My custom domain mail address is not the default when I write an email

Hi!

I brought my email account to mailbox.org, and as sucj I'd need to use my custom email "identity" as sender in all my emails. I appreciate the fact that there is a @mailbox.org address, but I don't want to use it, or give it to my contacts.

When I compose an email, I have to remember to switch sender, and more often than not, I don't. This is very annoying... is there a way to set the default sender?

Thanks a lot,

G.