how to send from custom domain address on iOS mail

I've created several external email aliases using my custom domain.

In the web app everything works as expected, I can receive and send from/to all normal and custom domain addresses.

I've setup apple Mail client on iOS an I can receive to @mailbox.org address and to my @custom-address.com addresses.

However I can't find a way to SEND from a custom domain address from apple Mail.

I can edit the FROM field in Thunderbird and it works, even if is not the most refined experience.

How can I solve this?