Using classic Outlook for Windows in business environments
Encountering this issue as well, no solution yet since we have no access to make these low level changes, but in my case it's due to an update to the software used by our e-mail provider.
Here's some reference to the issue:
https://support.cpanel.net/hc/en-us/articles/4420121088919-Exim-4-95-message-has-lines-too-long-for-transport-Error https://forums.cpanel.net/threads/problem-cpanel-39824-message-has-lines-too-long-for-transport-since-january-21-2022.697449/