Your Outlook email profile contains multiple email accounts.
For example, you may have user@example.com and user@example.net with [email protected] appearing first in the list and set to send from this account by default.
Even though Outlook sends from user@example.com, Desktop Email Encryption sends from user@example.net.
PGP Desktop Email Encryption 10.5 and above (Symantec Encryption Desktop)
When first configuring Outlook you added the [email protected] account first, even though you then added the [email protected] account and made it primary.
Create a new Outlook profile and first add the email account that you want Desktop Email Encryption to send from before adding any additional accounts.