[fetchmail] Multi-drop distribution to external server

Hi!

I could not solve the following problem:
I have a multi-drop *(_at_)multidrop(_dot_)example(_dot_) I want to
redistribute them by user name. The account has user
multiuser(_at_)mail(_dot_)provider(_dot_)example with password ***.

There are two addresses I want to handle:

user1(_at_)multidrop(_dot_)example -> user1(_at_)external(_dot_)example
user2(_at_)multidrop(_dot_)example -> user2(_at_)external(_dot_)example
(external.example is *not* the machine running fetchmail!)

All other mails should go to user2(_at_)external(_dot_)example(_dot_)

So this is in my .fetchmailrc:
poll mail.provider.example aka provider.example localdomains
multidrop.example: username multiuser password *** to
'user1'='user1(_at_)external(_dot_)example'
'user2'='user2(_at_)external(_dot_)example'

I call this by:
fetchmail --postmaster user2(_at_)external(_dot_)example
mail.provider.example

What happens is that recognized recipients
user[12](_at_)multidrop(_dot_)example are delivered to
user[12](_at_)multidrop(_dot_)example(_dot_) The rest goes -- as expected --
to user2(_at_)external(_dot_)example(_dot_)

So the problem is that mails for user1(_at_)multidrop(_dot_)example
don't go to user1(_at_)external(_dot_)example but to itself (AKA mail
loop).

What is my mistake and how do I achieve the right thing?

TIA. pi