Sending mails to a mailing list with sender restrictions from an external email address

hello,

is there a way to allow sending mails from a specific external email address to a mailing list with sender restrictions?

i know how it works with internal users/members of the mailing list but if i try it with an external email address (even if this email is a member of this mailing list) i get this error:

550 5.0.350 Remote server returned an error -> 550 Address unknown

Can someone tell me how to set this up or is this even possible?

p.s: i only have domain-admin permissions for icewarp therefore i have to consult my hoster for changes i can’t do on my own.