Roundcube does not allow attachments larger than 75MB on RPM-based operating systems

Symptoms

When using an RPM-based operating system such as AlmaLinux, after making the required changes to internal PHP post_max_size and upload_max_filesize settings along with the Exim message_size_limit setting, Roundcube still limits attachment size to 75MB.

Cause

It is not currently possible to allow attachments over 75MB in size for newly composed messages in Roundcube. 

Resolution

1. Increase "cPanel PHP max POST size" in WHM's Tweak Settings interface.
2. Increase "cPanel PHP max upload size" in WHM's Tweak Settings interface.
3. Increase message_size_limit in WHM's Exim Configuration Manager.
4. Increase the "max_message_size" value in /usr/local/cpanel/base/3rdparty/roundcube/config/config.inc.php.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-48108. Follow this article to receive an email notification when a solution is published in the product. 

Additional Resources

How to increase the maximum allowed attachment size in Webmail