Hi Aki, I work on the thunderbird (TB) imap code and am not sure if this problem is maybe something I've done wrong. Please see the information here: https://bugzilla.mozilla.org/show_bug.cgi?id=1968384#c10 TB is known to work OK doing APPEND using gmail and courier which support UTF8=ACCEPT but don't support LITERAL+. So dovecot 2.4 is the first server that I've seen and used with TB that supports both UTF8=ACCEPT and LITERAL+. When user Chris disables either UTF8=ACCEPT or LITERAL+ in TB it works OK with dovecot 2.4. But when both are enabled, dovecot 2.4 rejects the APPEND data (as you can see in the above logs). LITERAL+ just adds a "+" to the length value in parenthesis. With LITERAL+ disabled the "+" is not included and the length is the same. Should maybe the length be increased by 1 when the "+" is added? Or maybe TB is doing something else wrong? Note: I don't currently have an installation of dovecot 2.4 to test with locally. I currently only have 2.3 from fedora 42. --gene smith