If I understand what you're trying to do correctly, try this:
long press on account (in account list) > Options and folders > Composing and sending > Reply to <myname@domain.com>
No, that would still have "From: you@gmail.com".
The correct solution would be:
1. Long-press on the account -> "Account setup" -> "Manual". -> On each screen note the current value, then before "finish", go back to the beginning (or finish and start over.
Change "From" to username@domain.com
Then use the rest of the values as they were.
2. In addition you can (optionally): Change the SMTP server to your company's one.
Alternatively to this, if you have a paid app, you can also add an identity to that account with the settings as in #1 and, optionally in #2.
Also, see this pinned thread:
https://www.aqua-mail.com/forum/index.php?topic=1325.0