net/smtp: Add support for SMTPUTF8 extension (rfc6531)

[horkhe]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7.5 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/maxim/workspace/goworld"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/vr/lfwfw4nn5pz_w5203lt5_8140000gn/T/go-build690204281=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

Try sending an email to an address that contains UTF-8 characters.

What did you expect to see?

An email should be successfully delivered.

What did you see instead?

Delivery fails. The exact SMTP status code and message depend on the target server. E.g. gmail returns: 555 5.5.2 Syntax error. u200si18377193qka.138 - gsmtp

Even though gmail SMTP server advertise SMTPUTF8 in their EHLO response:

250-mx.google.com at your service, [157.130.196.214]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8

It expects to see SMTPUTF8 parameter in MAIL command, as per rfc6531:

One OPTIONAL parameter, SMTPUTF8, is added to the MAIL command.
The parameter does not accept a value. If this parameter is set
in the MAIL command, it indicates that the SMTP client is
SMTPUTF8-aware. Its presence also asserts that the envelope
includes the non-ASCII address, the message being sent is an
internationalized message, or the message being sent needs the
SMTPUTF8 support

[bradfitz]

See the package doc for net/smtp which says:

The smtp package is frozen and is not accepting new features. Some external packages provide more functionality. See:

https://godoc.org/?q=smtp

Have you verified there isn't a better smtp client implementation elsewhere?

Maybe you can own one?

[dcormier]

Given that other proposals for net/smtp are now being considered (at least #40034), perhaps this is worth another look? I've forked net/smtp to be able to have this functionality, but I'd really rather not maintain that.

[ianlancetaylor]

How big is the change?

[dcormier]

Very small.

[ianlancetaylor]

Sure, send it, we can take a look. Thanks.

[gopherbot]

Change https://golang.org/cl/247257 mentions this issue: net/smtp: adds support for the SMTPUTF8 extenstion