One vexing problem for users with multiple email accounts: when sending mail from other applications, such as accounting or invoicing applications, the message will be sent from the email account associated with the default data file, which may not be the default email account or even the account you want to send the messages from. Because the messages are generated and sent automatically, you can’t change the sending account.
In the profile used in this screenshot, messages would be sent from the Outlook.com account, not from the Gmail account, even though Gmail is set as the default email account.
The recommended solution is to create a new profile with only that account in the profile then switch profiles before sending messages using the software. While you could change the default data file to one used by the desired account, using a new profile insures the messages are sent from the correct account.