Exim fails to send email: message has lines too long for transport

Symptoms

You are unable to send emails, and you find a message similar to one of the following in the Exim mainlog.

2022-03-22 08:53:12 1nWfsm-0004op-5k ** user@domain.tld R=dkim_lookuphost T=dkim_remote_smtp: message has lines too long for transport

2022-11-10 13:20:45 1ot2nl-0002E5-U2 ** user@domain.tld R=dkim_lookuphost T=dkim_remote_smtp H=mail.domain.tld [203.0.113.2] X=TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=no: SMTP error from remote mail server after end of data: 500 Line too long

Description

Some email clients ignore RFC 5322 and send emails with lines exceeding the 998-character limit. Modifying the message_linelength_limit value allows Exim to accept messages exceeding the 998-character limit.

Workaround

1. Log into WHM as the ‘root’ user.
2. Navigate to "Home / Service Configuration / Exim Configuration Manager."
3. Click the "Mail" tab.
4. Scroll down to the "Maximum line length for SMTP transports" option.
5. Select the "Maximum line length for SMTP transports" textbox.
6. Enter the desired value in the textbox.
7. Click the "Save" button.

Additional rescources

Exim Configuration Manager Basic Editor: Mail