Use sender_canonical_maps to rewrite a sender during Postfix cleanup, rebuild its lookup table with postmap, and reload Postfix. To change only the SMTP envelope sender while leaving the visible From: header untouched, set sender_canonical_classes = envelope_sender. Use smtp_generic_maps instead when the rewrite should happen only as mail leaves the host through SMTP.
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Envelope sender and the visible From address are different
The envelope sender is the address Postfix supplies in the SMTP MAIL FROM command. Receiving systems use it for delivery decisions, bounce handling and other transport functions. The message also contains a visible From: header, which mail clients display to readers. Changing one does not automatically change the other.
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A receiving server may expose the envelope sender as a Return-Path: header after delivery. Editing the message’s visible From: header is therefore not a reliable way to change the address used for bounces. Configure the SMTP envelope layer explicitly.
This distinction is the first diagnostic question: do you want to change the transport identity used by MAIL FROM, the author identity displayed in the message, or both? Postfix offers different mapping stages and classes for those requirements. A successful header rewrite can still leave the SMTP envelope unchanged, and a successful envelope rewrite can intentionally leave the reader-facing header alone.
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Choose the mapping mechanism before editing files
| Mechanism | Where it runs | Addresses it can affect | Typical use |
|---|---|---|---|
sender_canonical_maps |
Postfix cleanup processing | Envelope sender and, by default, header sender; classes can be restricted | Sender-focused rewriting that should be applied as Postfix processes the message |
smtp_generic_maps |
Outbound SMTP delivery | Envelope and header addresses | Converting local or private addresses only when mail leaves this host |
Choose sender_canonical_maps when the requirement is specifically “rewrite this sender” and you need control over envelope and header classes. Choose smtp_generic_maps when the transformation belongs at the outbound SMTP client stage. The generic mapping is the outbound counterpart to canonical mapping and is non-recursive.
Neither setting is a substitute for the other. A cleanup-stage rule may affect messages before they are queued or delivered, while a generic rule is evaluated when Postfix delivers through SMTP. A service in master.cf can also disable address mappings, so a service-level exception may override a globally correct configuration.
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- Processing stage: canonical mapping runs during cleanup; generic mapping runs when the SMTP client delivers.
- Address scope: canonical classes can be narrowed to the envelope sender, while generic mapping affects envelope and header addresses.
- Direction: canonical rewriting belongs to normal message processing; generic rewriting is specifically an outbound transformation.
- Service behavior: a
master.cfservice usingno_address_mappingscan bypass mappings that are active elsewhere.
Method 1: rewrite with sender_canonical_maps
This is the usual sender-focused solution. The example below maps one original address to one replacement address. Replace the addresses with values valid for your environment.
1. Add the sender map in main.cf
Edit /etc/postfix/main.cf and add a supported table type. A documented LMDB example is:
sender_canonical_maps = lmdb:/etc/postfix/sender_canonicalInstallations without LMDB support can use another table type available on that system, such as:
sender_canonical_maps = hash:/etc/postfix/sender_canonicalThe database prefix must match the lookup-table support compiled into your Postfix installation. Keep the path consistent with the file you will edit in the next step. If postconf shows a different existing value, edit that configuration deliberately rather than creating competing definitions.
2. Create the source map
Create /etc/postfix/sender_canonical. Put the original sender on the left and the replacement on the right, separated by whitespace:
[email protected] [email protected]The left side is the address Postfix receives from the message. The right side is the address Postfix should use after the mapping is applied. Add additional explicit lines when more senders need different replacements. Avoid an unintended wildcard or catch-all entry while testing; a broad pattern can rewrite senders you did not intend to touch.
Keep the source file as the authoritative, human-readable copy. The indexed file created by postmap is derived data. When troubleshooting, confirm that you edited the same source path named in main.cf, rather than a similarly named file in another directory.
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3. Decide whether headers should change
By default, sender canonical mapping applies to both the envelope sender and header sender. To rewrite only the SMTP envelope sender, add this setting to /etc/postfix/main.cf:
sender_canonical_classes = envelope_senderWith that restriction, the MAIL FROM address is mapped but the message’s visible From: sender is left as it was. If you omit the setting, the default classes include envelope_sender and header_sender, so the same mapping can alter both layers.
Use the default only when changing the visible sender is intentional. If users must continue seeing the original author address while bounces go to a different mailbox, the envelope-only setting is the important part of the configuration. Conversely, do not select envelope-only behavior if the operational requirement is to make both the transport and visible sender consistent.
4. Build the indexed lookup table
After creating or changing the source file, run postmap against that exact path:
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5. Reload Postfix
postfix reloadA reload makes the changed configuration visible without waiting for the normal configuration delay and does not require stopping the mail service. Keep the table rebuild and reload together in your change procedure so that a corrected map is actually active.
What canonical mapping changes in practice
Sender-specific mappings are applied before common canonical_maps. That ordering helps keep a sender-only requirement from unintentionally becoming a broader canonical rewrite that also changes recipients. It also means you should inspect any existing canonical configuration when a result looks wider than expected: the sender map may be correct while a later or common rule accounts for another change.
Method 2: rewrite at outbound SMTP time with smtp_generic_maps
Use a generic map when the address should be transformed only as Postfix sends mail through its SMTP client. This is useful when software submits mail with a local-only domain but external recipients must see a publicly deliverable address.
Configure the generic map
Add the map to /etc/postfix/main.cf:
smtp_generic_maps = hash:/etc/postfix/genericCreate /etc/postfix/generic with a left-to-right mapping:
[email protected] [email protected]Then compile and activate it:
postmap /etc/postfix/generic
postfix reloadThe generic mapping replaces envelope and header addresses during outbound SMTP delivery. It is non-recursive, so Postfix does not repeatedly feed a replacement back through the same mapping. Because it affects both address layers at that delivery stage, it is not the right choice when the visible From: header must remain unchanged.
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This outbound timing is useful when the local representation is acceptable inside the host or queue but is not suitable for the next SMTP server. The address presented at the external boundary can consequently differ from the address used earlier in processing. If the requirement is to change the sender before queuing or to preserve a header independently, use the canonical mechanism instead.
How the processing stages affect the result
Cleanup-stage canonical rewriting
sender_canonical_maps is evaluated in Postfix cleanup processing. Sender-specific mappings are applied before common canonical_maps, which helps keep a sender-only requirement from unintentionally turning into a broader recipient rewrite. The result can travel with the queued message and be used by later delivery stages.
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Because the mapping is associated with cleanup, test messages submitted through different services can have different results if one service disables address mappings. Do not assume that a map tested through local pickup was also used by a submission service or another path.
Outbound SMTP generic rewriting
smtp_generic_maps is evaluated when Postfix is acting as an SMTP client for delivery. The address used internally before delivery can therefore differ from the address presented to the next SMTP server. This stage-specific behavior is why generic mapping is suited to translating local addresses only at the external boundary.
Generic mapping is non-recursive. A replacement is not repeatedly looked up as another source value in the same generic table. Define the intended final address directly in the right-hand side of the entry.
Service-level bypasses
A service definition in master.cf may include:
receive_override_options=no_address_mappingsWhen that option is present for the service receiving the message, address mappings can be disabled there. A map that works for ordinary mail can consequently appear ineffective for one submission, pickup or other service. Compare the service path used by the test message with the relevant master.cf entry before changing a working global map.
This is a scope issue, not necessarily a syntax issue. First identify how the failing message entered Postfix, then inspect that service’s override options. Changing the global map cannot affect a service that explicitly bypasses address mappings.
Verify the active configuration and the actual envelope sender
Verification should distinguish configuration state from the address that a receiving server actually sees.
- Check which mechanism is active. For canonical rewriting, run
postconf sender_canonical_maps sender_canonical_classes. For outbound rewriting, runpostconf smtp_generic_maps. - Open the source map and confirm the intended original address is on the left and replacement address on the right. Check for spelling, whitespace errors and unintended wildcard entries.
- Run
postmapagainst the exact source file after the last edit. A changed text file without a rebuilt table is not an active change. - Run
postfix reloadafter rebuilding the table. - Send a controlled test message through the same Postfix service and delivery path used in production.
- Inspect the receiving server’s SMTP transaction, delivered headers or displayed
Return-Path. Confirm the envelope sender changed, and separately confirm whether the visibleFrom:header changed. A header-only inspection cannot prove what was sent inMAIL FROM.
For an envelope-only canonical rule, the expected result is a new envelope sender with the original visible From: value. For a generic rule, expect both envelope and header addresses to reflect the outbound mapping when the mapped address appears in each layer.
Use a newly submitted controlled message for verification. A message already queued may have gone through an earlier processing state, so it is not a reliable test of a newly rebuilt map and reloaded configuration.
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Troubleshoot a mapping that appears not to work
The configuration shows the old value
- Run the appropriate
postconfcommand and verify that the map parameter points to the file you edited. - Check that the table prefix, such as
lmdb:orhash:, is supported by this Postfix installation. - Re-run
postmapusing the exact source path, then reload Postfix.
The envelope changed, but the From header did not—or vice versa
Check sender_canonical_classes. The default includes both envelope_sender and header_sender; setting it to envelope_sender deliberately limits the rewrite. If you selected smtp_generic_maps, remember that its outbound mapping affects both envelope and header addresses, so it cannot provide envelope-only behavior.
Do not infer the envelope value from a displayed header. Confirm the SMTP transaction or the receiving server’s resulting Return-Path, then inspect the visible From: separately.
A rule works for one mail path but not another
Inspect the service in master.cf. The no_address_mappings override can bypass address mappings for a particular service even when the global configuration is correct. Repeat the test through the exact service that failed.
The source file was edited but delivery is unchanged
The indexed database may still contain the previous contents. Rebuild it with postmap, reload Postfix, and send a new controlled message. Existing mail already queued may have been processed under the earlier state, so test a newly submitted message.
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A recipient reports the wrong Return-Path
Check the envelope transaction rather than changing only the visible header. The address supplied in MAIL FROM is the value that governs bounce routing. Use the receiving server’s transaction details or delivered message metadata to determine whether the intended envelope mapping was applied.
The map appears to rewrite too much
Review the source file for a wildcard or broad pattern and verify whether the default canonical classes are rewriting header senders as well as envelope senders. An explicit entry and sender_canonical_classes = envelope_sender provide a narrower test. Also inspect common canonical_maps, because sender-specific mappings are processed before those common mappings.
Practical configuration patterns
Keep the visible author address, change only bounces
Use canonical mapping with the envelope class restricted:
# /etc/postfix/main.cf
sender_canonical_maps = hash:/etc/postfix/sender_canonical
sender_canonical_classes = envelope_sender# /etc/postfix/sender_canonical
[email protected] [email protected]postmap /etc/postfix/sender_canonical
postfix reloadThis pattern separates the transport sender from the author identity shown in the message.
Translate a local submission address for external SMTP
Use a generic map when the rewrite belongs only at the outbound boundary:
# /etc/postfix/main.cf
smtp_generic_maps = hash:/etc/postfix/generic# /etc/postfix/generic
[email protected] [email protected]postmap /etc/postfix/generic
postfix reloadThis changes the envelope and header forms presented during outbound SMTP delivery.
Safe change and rollback checklist
- Back up the current
main.cfand map file before editing. - Make one mechanism responsible for the requirement: canonical mapping for cleanup-stage sender control, or generic mapping for outbound SMTP translation.
- Use an explicit sender entry while testing instead of a broad pattern.
- Rebuild the indexed table after each source-file change.
- Reload Postfix and verify with a newly submitted test message.
- To roll back, restore the previous map and configuration values, run
postmapon the restored source file, and executepostfix reload.
A rollback is incomplete if only the text file is restored. The indexed table must also be rebuilt, and Postfix must be reloaded so the restored configuration and lookup data are visible to subsequent messages.
Decision summary
| Requirement | Configuration choice | Important qualification |
|---|---|---|
| Change the envelope sender during normal Postfix processing | sender_canonical_maps |
Rebuild with postmap and reload |
Change only MAIL FROM while preserving visible From: |
sender_canonical_maps plus sender_canonical_classes = envelope_sender |
The default classes also include header_sender |
| Rewrite addresses only as mail leaves through SMTP | smtp_generic_maps |
Outbound mapping affects envelope and header addresses and is non-recursive |
| One service ignores an otherwise valid map | Inspect that service’s master.cf entry |
receive_override_options=no_address_mappings can bypass mappings |
Frequently Asked Questions
How do I change only the Postfix envelope-from address?
Configure sender_canonical_maps, add the original and replacement addresses to the map, and set sender_canonical_classes = envelope_sender. Rebuild the table with postmap and run postfix reload. The visible From: header remains unchanged.
How do I change the Return-Path in Postfix?
Change the SMTP envelope sender, not only the visible header. The envelope sender is supplied in the MAIL FROM command and may appear as Return-Path: after delivery. Use a canonical sender map for normal processing, or a generic map for outbound SMTP rewriting.
What is the difference between MAIL FROM and From:?
MAIL FROM is part of the SMTP envelope and is used for delivery and bounces. From: is a message header displayed by mail clients. They are separate layers, so changing the header alone does not change the bounce address.
Should I use sender_canonical_maps or smtp_generic_maps?
Use sender_canonical_maps for sender-focused cleanup rewriting and control it with sender classes. Use smtp_generic_maps when the transformation should occur only as Postfix delivers through SMTP, such as converting a local-only domain for external delivery.
Why did my canonical map change the visible From header too?
The default sender canonical classes include both envelope_sender and header_sender. Set sender_canonical_classes = envelope_sender when the visible header must remain unchanged.
Does smtp_generic_maps support envelope-only rewriting?
No. The generic mapping replaces envelope and header addresses during outbound SMTP delivery. It is therefore unsuitable when you must change MAIL FROM while preserving the visible From: header.
Do I need to run postmap after editing the map?
Yes. Postfix reads the indexed lookup database generated by postmap. Run postmap against the exact source file after every change, then reload Postfix so the active configuration sees the rebuilt data.
Is a Postfix restart required?
Use postfix reload after rebuilding the map. A reload makes configuration changes visible without stopping the mail service or waiting for the normal configuration delay.
Why does the map work for one service but not another?
Check the relevant master.cf service. A service can specify receive_override_options=no_address_mappings, which bypasses address mappings for messages entering through that path.
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How can I verify the actual envelope sender?
Send a controlled new message through the same service and delivery path, then inspect the receiving server’s SMTP transaction, delivered metadata or resulting Return-Path. A visible From: header alone cannot prove which address was used in MAIL FROM.
The Bottom Line
For a Postfix envelope-from rewrite, use sender_canonical_maps, rebuild the map with postmap, and reload Postfix. Add sender_canonical_classes = envelope_sender when only MAIL FROM should change. Choose smtp_generic_maps for outbound-only rewriting, and always verify the receiving server’s envelope transaction separately from the visible From: header.
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