Noticed for one emailaccount that in /home/<user>/imap/<domain>/<account>/Maildir there are ".Sent Messages" and .Send folders for send emails. All other accounts have only .Send. Where is this ".Sent Messages" folder come from? Where is this used for?
Different IMAP clients use different naming conventions for a Sent folder. Apple's Mail.app on both Mac and IOS, and I think Outlook use "Sent Messages" by default for the Sent folder. Your clients are the ones that creates those folders.