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). Introduced with Postfix version 2.12, this fully supports UTF-8 email addresses and UTF-8 message header values.

Topics covered in this document:

Building Postfix with/without SMTPUTF8 support

Postfix will build with SMTPUTF8 support if the ICU library and header files are installed on the system. The package name varies with the OS distribution (and version). The table shows package names for a number of platforms at the time this text was written.

OS Distribution Package
FreeBSD, NetBSD, etc. icu
Centos, Fedora, RHEL libicu-devel
Debian, Ubuntu libicu-dev

To force Postfix to build without SMTPUTF8, specify:

$ make makefiles CCARGS=-DNO_EAI ...

See the INSTALL document for more "make makefiles" options.

Enabling Postfix SMTPUTF8 support

There is more to SMTPUTF8 than just Postfix itself. The rest of your email infrastructure also needs to be able to handle UTF-8 email addresses and message header values. This includes SMTPUTF8 protocol support in SMTP-based content filters (Amavisd), LMTP servers (Dovecot), and down-stream SMTP servers.

Postfix SMTPUTF8 support is enabled by default, but it may be disabled as part of a backwards-compatibility safety net (see the Postfix 2.12 RELEASE_NOTES file).

SMTPUTF8 support is enabled by setting the smtputf8_enable parameter in main.cf:

# postconf "smtputf8_enable = yes"
# postfix reload

With SMTPUTF8 support enabled, Postfix changes behavior with respect to earlier Postfix releases:

Postfix already permitted UTF-8 in message header values and in address localparts. This does not change.

Using Postfix SMTPUTF8 support

After Postfix SMTPUTF8 support is turned on, Postfix behavior will depend on 1) whether a remote SMTP client requests SMTPUTF8 support, 2) the presence of UTF-8 content in the message envelope and headers, and 3) whether a down-stream SMTP (or LMTP) server announces SMTPUTF8 support.

SMTPUTF8 autodetection

This section applies only to systems that have SMTPUTF8 support turned on (smtputf8_enable = yes).

For compatibility with pre-SMTPUTF8 environments, Postfix does not automatically set the "SMTPUTF8 requested" flag on messages from non-SMTPUTF8 clients that contain an UTF-8 header value or UTF-8 address localpart. This would make such messages undeliverable to non-SMTPUTF8 servers, and could be a barrier to SMTPUTF8 adoption.

By default, Postfix sets the "SMTPUTF8 requested" flag only 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

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

/etc/postfix/main.cf:
    smtputf8_autodetect_classes = all

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

Limitations of the current implementation

The Postfix implementation is a work in progress; limitations are steadily being removed. The text below describes the situation at one point in time.

No automatic conversions between ASCII and UTF-8 domain names.

Some background: According to RFC 6530 and related documents, "Internationalized" domain names can appear in two forms: the UTF-8 form, and the ASCII (xn--mumble) form. "Internationalized" address localparts must be encoded in UTF-8; the RFCs do not define an ASCII form for the same information.

Postfix currently does not convert internationalized domain names from UTF-8 into ASCII (or from ASCII into UTF-8) before using domain names in SMTP commands and responses, before looking up domain names in mydestination, relay_domains, access tables, etc., before using domain names in a policy daemon or Milter request, or before logging domain names.

Postfix does, however, casefold domain names and email addresses before matching them against a Postfix configuration parameter or lookup table.

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 may have to apply RFC 2047 encoding to satisfy that last condition.

Pre-existing non-ASCII email flows

With "smtputf8_enable = no", Postfix handles email with non-ASCII in address localparts (and in headers) as before. The vast majority of email software is perfectly capable of handling such email, even if pre-SMTPUTF8 standards do not support such practice.

However, when you specify "smtputf8_enable = yes", Postfix requires that non-ASCII address information is encoded in UTF-8 and will reject other encodings such as ISO-8859. It is not practical for Postfix to support multiple encodings at the same time. There is no problem with RFC 2047 encodings such as "=?ISO-8859-1?Q?text?=", because those use only characters from the ASCII characterset.

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