Mail service integration with Lotus Notes Domino

Welcome to the forums.

In the most general of terms, smtp is smtp, imap is imap, pop is pop, still ugly, and best avoided in favor of imap, and they interoperate.

As for your case, you face some choices...

...either run your local group's mail on your Mac OS X Server (which you apparently cannot do here?) and set up the corporate DNS and configure the corporate employee directory (LDAP, Microsoft GAL, whatever) to forward mail to your mail server...

...or you need to configure your mail clients to use your organization's smtp/pop/imap server running within your target environment (and get the smtp/pop/imap support enabled, if it's not already enabled)...

...or you need to figure out how to break this protocol and mail server and version deadlock with your management...

Lotus/Domino has had SMTP/POP/IMAP services for a decade or so, and there are various postings around the 'net reporting successful connections from Apple mail into Lotus/Domino servers, so your local Mac OS X clients should connect into most any Domino server around, if it's enabled for any of the "standard" mail clients that are in use, including Apple's Mail.app, Mozilla's Thunderbird and Seahorse, Microsoft's Outlook Express, etc.