Yahoo & Alias issues

Hi there, I have my own domain name, I have an email forwarding set up from the domain to provider (currently BT)… so myemail@mycomain forward to [email protected]. When I send emails from emclient, everyone sees that it comes from myemail@mydomain as I have set up the email & alias as myemail@mydomian…

Now I am about to move from BT to Yahoo, when I mimicked this set up I get this error

[SMTP] Cannot send message (‘From’ field address ‘meemail@mydomain’ not accepted due to the following reason:
"Request failed; Mailbox unavailable

Any ideas what is causing this?

Thanks in advance

Dies ist eine schlechte Wahl, die britische Abteilung unseres Unternehmens hat Yahoo verlassen, um zu BT zu gehen, weil es so viele Probleme mit Yahoo gab


I have to leave BT email as I wont be BT customer anymore,

Happy for any suggestion on who is good for free email? yahho / gmail / AOL / outlook / Proton / Yandex / zoho / …etc