Upon updating to cPanel version 102 that includes Exim 4.95, under certain circumstances outbound email bounces with an error similar to the following:
1nBxPK-0023Ax-M1 ** email@example.com R=dkim_lookuphost T=dkim_remote_smtp: message has lines too long for transport
Headers or other lines in the email are longer than the character limit which triggers this error.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-39824. Follow this article to receive an email notification when a solution is published in the product.
The issue may be related to sending emails via Outlook. The default message_linelength_limit was increased, and an option to adjust this limit was added to the Exim Configuration Manager in cPanel version 102.0.2. This setting is called "Maximum line length for SMTP transports"
We have identified additional scenarios for this error: