Account email different to mailbox username?

Have just installed K9 Mail on a Galaxy S5, Android version 6.0.1

To hide the name of an email account/box on a server, I use different names. So I can’t see how to set this up in K9 Mail.

For example, server username is fredbloggs-mailbox@example.com
K9 Mail username is obviously the same for POP3 and SMTP, and it works.

But where do I setup a default email address of the format fredbloggs@example.com

(I have email forwarders setup on the server of course)

Settings > [Account] > Sending mail > Manage identities

If you don’t use fredbloggs-mailbox@example.com you can just change the initial identity. The usernames for POP3 and SMTP are stored separately (and can be edited under Settings > [Account] > Fetching mail > Incoming server > Username and Settings > [Account] > Sending mail > Outgoing server > Username).

Thanks for your reply