Share internet domain name with different systems

[cparker33]

After a merger of companies, I need to know how to share one internet domain name among 4 different email systems.

I know how to route incoming internet mail from our SMTP gateway to each site, but am not sure how to route internal mail.
For example:
Site A - running Groupwise 7.02
Site B - running Exchange 2003
Site C - running Exchange 2003

all sites using internet domain of abc.com and sharing a common SMTP gateway/MX record.

How can I route mail between facilities running separate email systems?

Thanks in advance..