This is a known problem that is caused when you have multiple pst files and it always defaults to the POP3 account rather than the IMAP, even if you specify the IMAP account as default. I have seen workarounds for PCs that involve adding a registry entry but none for Mac. I had 2011 installed when I posted this but have recently upgraded to 2016 and yes, all updates are loaded.
Sep 04, 2013 (default) Outlook uses the pre-SP1 behavior and determines the sending account for new outbound email messages based on the account in which the user is working. The default sending account is ignored for Outlook mail but is used for Mail to and Simple MAPI operations. Choose Preferences from the Mail menu, then click General. Or, if you're prompted to set up an email account, add your account, then choose Preferences from the Mail menu.* Choose your email app from the ”Default email reader” pop-up menu. * You can also set your default email app in the preferences of your third-party email app.
There is no error given. Every time I compose a new email, it defaults the send account to my POP3 account (charter.com). I only use that account to receive mail, and very rarely send from it. I want to send from my icloud.com account which is set up IMAP.