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 the following in the Exim main_log:

CONFIG_TEXT: 1nWfsm-0004op-5k ** user@domain.tld R=dkim_lookuphost T=dkim_remote_smtp: message has lines too long for transport

Description

RFC-5322 defines that a single line in an email header cannot be more than 998 characters. However, some email clients and mail sending applications may ignore RFC-5322, and send emails with lines exceeding this 998 character limit. 

Workaround

Increase the message_linelength_limit value allows Exim to accept messages exceeding the 998-character limit.

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. Enter the desired value in the textbox.
6. Click the "Save" button at the bottom of the page.

Note: While increasing this limit will allow your server to send messages with an increased character count in the headers, remote servers may still enforce this limit and reject the message.

Additional Resources

Exim Configuration Manager Basic Editor: Mail

"Message has lines too long for transport" with custom routers