I am using Windows Mail on Windows 10 Pro, and find that several emails are deleted from the sent items list right after I send them. It happens inconsistently, but apparently with certain contacts, in particular. Right after I send the email, it appears in "Sent Items" for a second, and then vanishes. It is nowhere to be found. The incoming server is IMAP, the outoing SMTP. All boxes are ticked in the account settings for this account (Outgoing server requies authentication; Use the same username and  password for sending email; Require SSL for incoming email, Require SSL for outgoing email).

Can anyone help?