#1955 SMTP receiver rejects RFC 2047 Subject encoded as ISO-8859-2

SMTP publishing fails on RFC 2047 Subject encoded as ISO-8859-2

Description

ntfy's SMTP receiver rejects an otherwise valid e-mail when the Subject header uses RFC 2047 encoding with the ISO-8859-2 charset.

The SMTP server returns:

554 5.0.0 Error: transaction failed, blame it on the weather: mime: unhandled charset "ISO-8859-2"

This was encountered with a Brother MFC-L2710DW printer configured to send e-mail notifications directly to ntfy.

When the printer's local language is Polish, it generates a Subject using ISO-8859-2, and ntfy rejects the message. Changing the printer's local language to English makes the same SMTP test succeed.

Reproduction

Configure ntfy's built-in SMTP receiver.
Send an e-mail containing an RFC 2047 encoded Subject using ISO-8859-2, for example a header of the form:

Subject: =?ISO-8859-2?Q?...?=

ntfy rejects the SMTP DATA transaction with:

554 5.0.0 Error: transaction failed, blame it on the weather: mime: unhandled charset "ISO-8859-2"

Expected behavior

ntfy should decode valid MIME/RFC 2047 legacy character sets such as ISO-8859-2 to UTF-8 and publish the message.

At minimum, a charset used by ordinary legacy network appliances should not cause the entire SMTP transaction to fail.

Actual behavior

The message is rejected with SMTP status 554.

Suspected cause

In server/smtp_server.go, the Subject is currently decoded using:

dec := mime.WordDecoder{}
subject, err := dec.DecodeHeader(subject)

mime.WordDecoder requires a CharsetReader to decode character sets other than those supported natively by Go's mime package.

A possible fix would be to provide an appropriate charset reader and convert decoded header values to UTF-8.

It may also be worth checking text MIME body handling for the same issue, since the current SMTP parser appears to process transfer encoding but not the charset= parameter of text/plain / text/html parts.

Environment

Self-hosted ntfy
SMTP publishing enabled
Sender: Brother MFC-L2710DW
Printer language when failing: Polish
Printer language when succeeding: English
Failing charset: ISO-8859-2

ntfy version: 2.28.0