Replies are not stored in sent folder

Does this happen in a POP or an IMAP account? Any error messages? Do you know whether the sent messages are stored in Local Folders?

IMAP account No error message Sent messages are set to store in sent folder of the email

Try this: in Account Settings > your account name > Copies & Folders : instead of the default setting check ◉ Other : Sent on your account name (see image) Sometimes this helps ....