Exim returns the error "message has lines too long for transport" when sending a message

Symptoms

The Exim mail server does not send emails, there is a format error in the server error log:

<email-id> ** <email-addressee> R=dnslookup T=remote_smtp: message has lines too long for transportation

Causes

The message text contains a string with a number of characters that, in total bytes, exceeds the limit of the message_linelength_limit parameter.

Solution

The best solution is to address the problematic message: identify and edit the problematic line.

If it is not possible to do so, you can increase the value of the message_linelength_limit parameter:

1. Using any text editor, open the configuration file /etc/exim4/exim4.conf.template (/etc/exim/exim.conf for CentOS and AlmaLinux);
2. Go to the remote_smtp section on the line after the driver = smtp parameter;
3. By default, message_linelength_limit is not written in the configuration file, so it must be specified along with the required value, e.g., message_linelength_limit=10000.