We have an on-site Exchange 2016 server and users using Outlook 2016. They are both fully patched.
If a managers secretary creates an email and saves it in her managers draft folder for checking (The secretary has full rights to this folder). When she then goes to send it, it bounces back with a IMCEAINVALID error. Looking closer the secretary is creating the email using her managers "From" address and when it is saved in her mangers draft folder if the To address email properties are examined the email address has become the display name in single quotes ('firstname.lastname@example.org'), there is no email address and no email type. If the secretary saves it to her own draft folder it is fine. If the secretary puts her address as the "from" address and saves it in her managers draft folder it still strips out the To: email address and type. In both cases, if we then add the email address and type manually whilst it is in the managers draft folder the email then sends. Any ideas would be very much appreciated. Many thanks.