[exim] aliasing of another domain.

Many (most but not all) users in my-main-domain have also
an alias as
name.surname : username
most of them have also
name.surname@otherdomain forwarded to name.surname@my-main-domain
this is al ok. I wish however that:
a. if the message is sent to name.surname@otherdomain a list of aliases
be checked to see if the addresse is in one alis list, so the message is
not sent out and then bounced back;
b. if the message is sent to any.one@otherdomain if the sender is in the
list of aliases the from field be mangled to become
name.surname@otherdomain

suggestions ?