If someone in our organization emails a list of outside addresses
and CC's a few people within our organization the messages go
through. But if one of the outside recipients replies to all people,
the CC's are bounced back as bad addresses.

The sender's addresses shows as "<username>@domain.com"
but the CC's show as "<username>.<PostOffice>.<Domain>@domain.com".

Is there something that we should be doing to adjust how internal
addresses are reported on emails?