Postfix SMTPUTF8 support


Overview

This document describes Postfix support for Email Address Internationalization (EAI) as defined in RFC 6531 (SMTPUTF8 extension), RFC 6532 (Internationalized email headers) and RFC 6533 (Internationalized delivery status notifications). This fully supports UTF-8 email addresses and UTF-8 message header values.

Topics covered in this document:

Enabling Postfix SMTPUTF8 support

By default, SMTPUTF8 support is disabled, and Postfix should work exactly as it has worked before.

SMTPUTF8 support is enabled by setting in main.cf:

/etc/postfix/main.cf:
    smtputf8_enable = yes

and by issuing the "postfix reload" command.

With SMTPUTF8 support enabled, Postfix changes behavior as follows:

SMTP-based content filters (Amavisd) and LMTP servers (Dovecot) will need to be updated to support SMTPUTF8. A notice has been sent to the respective maintainers.

Using Postfix SMTPUTF8 support

SMTPUTF8 autodetection

By default, Postfix sets the "SMTPUTF8 requested" bit on address verification probes and on Postfix sendmail submissions that contain UTF-8 in the sender address, UTF-8 in a recipient address, or UTF-8 in a message header value.

/etc/postfix/main.cf:
    smtputf8_autodetect_classes = sendmail, verify

This will suffice for the vast majority of sites. It minimizes the chances of accidentally setting the "SMTPUTF8 requested" bit on a message from a non-SMTPUTF8 client, thereby making that message undeliverable to a non-SMTPUTF8 server as discussed above. Remember, once a message is flagged as "SMTPUTF8 requested", the Postfix SMTP client may not be able to deliver it to a non-SMTPUTF8 server.

However, if you have a non-ASCII myorigin or mydomain setting, or if you have virtual aliases, canonical mappings, or BCC mappings that introduce UTF-8 addresses, then you may have to apply SMTPUTF8 autodetection to all mail:

    smtputf8_autodetect_classes = all

This will, of course, also flag mail that was received without SMTPUTF8 request, but that contains UTF-8 in a sender address localpart, receiver address localpart, or message header value. Such mail was not standards-compliant, but Postfix would have delivered it if SMTPUTF8 support was disabled.

Limitations of the current implementation

"Internationalized" domain names can appear in two forms: the UTF-8 form, and the ASCII (xn--mumble) form.

No characterset canonicalization for non-ASCII domain names.

Postfix currently does not translate domain names from UTF-8 into ASCII (or ASCII into UTF-8) before looking up the domain name in mydestination, relay_domains, access tables, etc., before logging the domain name, or before using the domain name in a policy daemon or Milter request. You will have to configure both UTF-8 and ASCII forms in Postfix configuration files; and both forms will have to be handled by logfile tools, policy daemons and Milters.

No case canonicalization for non-ASCII characters.

Postfix currently does not case-fold non-ASCII characters when looking up an "Internationalized" domain name in mydestination, relay_domains, access maps, etc. Some non-ASCII scripts do not distinguish between upper and lower case, some have different numbers of upper and lower case characters.

Compatibility with pre-SMTPUTF8 environments

Mailing lists with UTF-8 and non-UTF-8 subscribers

With Postfix, there is no need to split mailing lists into UTF-8 and non-UTF-8 members. Postfix will try to deliver the non-UTF8 subscribers over "traditional" non-SMTPUTF8 sessions, as long as the message has an ASCII envelope sender address and all-ASCII header values. The mailing list manager will have to apply RFC 2047 encoding to satisfy that last condition.

Pre-existing non-ASCII mail flows

In pre-SMTPUTF8 environments, mail with UTF-8 in address localparts (and in headers) works just fine because the vast majority of mail software including Postfix is perfectly capable of handling such mail, even if pre-SMTPUTF8 standards do not support this.

Therefore, when Postfix SMTPUTF8 support is turned on, Postfix must not suddenly start to break pre-existing mail flows with UTF-8 in addres localparts (and in headers).

Thus, Postfix continues to permit UTF-8 in address localparts (and in headers) in mail from and to pre-SMTPUTF8 systems. At least, that is the default.

Building with/without SMTPUTF8 support

Postfix automatically builds with SMTPUTF8 support when the ICU library and header files are installed. To force Postfix to build without SMTPUTF8, specify:

$ make makefiles -DNO_EAI ...

Credits