[Exim] Sender Verification

Hi!

I have set sender_verify in my setup. The following problem occurred:

MX 10 could not verify the sender address, returned 451
The sending MTA immediately tried MX 20 (Ist that ok?)
MX 20 said relaying not allowed, returned 550 (this is our provider, seems I
have to do a phone call)

Sending MTA then gave up permanently. Is that ok? The highest prio MX (me)
said 451!
Is this a local or remote conf error?

Thanks!

Daniel