Reading and writing messages

When you connect the external account, you automaticaly can use the address of the connected account as the address of the sender.
 
Click on the address of the sender (From) in the window with the new message (next to the email address is small arrow) and select the required adress from the list.

If you reply or forward a message, the address of the sender will be selected automaticaly according to the mailbox, where you are situated.
 
Note:  Emails sent by using the identity of the external account are saved into the Sent folder of the main mailbox.