My account email is missing from my dropdown

I have multiple email accounts and one of my accounts receives emails, but when I got to send one, that account email address is not available in the drop down. Both IMAP and STMP are checked under services in the account settings … how can it be that’s in not available in the drop down?
When I tried to send from that inbox, it switches to the default email address as the sender (instead of that account’s email address).

I hope that makes sense and more importantly that you can help me. THANK YOU!

This is probably a Microsoft account (hotmail, outlook, live, msn etc.).

Please tick the AirSync service in the General tab, and click on Save & Close.

It is and that worked a charm - THANK YOU!!