In /var/log/maillog errors like the following may be seen.
spamd: razor2: razor2 check failed: Disk quota exceeded error closing log: Disk quota exceeded at /usr/local/cpanel/3rdparty/perl/530/lib/perl5/cpanel_lib/Mail/SpamAssassin/Plugin/Razor2.pm line 246, <GEN560> line 1. at /usr/local/cpanel/3rdparty/perl/530/lib/perl5/cpanel_lib/Mail/SpamAssassin/Plugin/Razor2.pm line 329.
SpamAssassin has a plugin called Razor2 that checks emails against external filters and writes data into the users .razor folder like /home/someuser/.razor and is written as that user.
Just after the log output, there should be a mailbox reported. Take the incoming domain's user and check to see if that user has exceeded disk quota. If it has the user's quota will need to be increased or data removed to free space.
An example command to check the account's quota from the command line would be:
quota -vs cpericd
Please note if new files are being created and inodes are restricted by user as well then the inode limit may need to be increased for that user as well.