if you send an email with the Bcc field, and look in the Sent folder, you will see Bcc like this:
Problem is in file: /classes/mail/imap.class.inc
where is missing mime_header_decode for bcc (like row 1117)
In attachment is diff patch.
GO version 3.7.28
Please feel free to include this modification, if you like.
Best Regards, Pavel Richter