emClient & Fastmail

Hi,

I would like to move from IONOS to Fastmail for my custom domain and email addresses.
However, if I switch to Fastmail, all of my custom domain email addresses will be part of a user (email/password account).

So I sent up an IMAP account for fastmail (main account), but obviously I can’t SEND emails using one of my custom domain email addresses because the only option is to send using the main account one, set up in emclient.

Is there a solution for this pls?