Thunderbird Version 115.10.1 on Windows 10 I wrote my own POP3 server and it largely seems to work apart from when Thunderbird tries to RETR the email messages at which point I get the error "Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk space to copy the mailbox.". If I drag a saved version of that same email that fails to be retrieved into Thunderbird , it can write it to the Inbox without issue. That means Thunderbird has permission to write to the Inbox file and so it is not a permission issue and there is enough space. I feel like it is more likely to be something I am doing wrong in my protocol implementation. For example, could I get this error if I misrepresented the octet count? Also I notice that even though my CAPA reply does not include UIDL, Thunderbird still seems to send that request and should get the response "-ERR Unknown command". Output from my server showing what Thunderbird sent to it: CAPA USER person@mydomain.com PASS XXXXX STAT LIST UIDL RETR 1 QUIT Any help is much appreciated, even if it is guidance on how I can debug it more thoroughly. Thanks.