Format of email content changes when sent between Domino servers on the same LAN

What are the other users setting for "format preferences for incoming mail"? Also what type of routing (Notes NRPC or SMTP) is enabled between server?

I believe all my users are set to "No Preference" in that field in the Public Name and Address book. I checked the few that I've been testing with to verify that it was the case.

You may be on to something with the routing. Of the 10 servers in our company, only 2 are actually set to "Mail Routing, SMTP Mail Routing" in the Routing Tasks field on their server documents. I'm going to set them to "SMTP Mail Routing" and retest....

I'll let you know

You need to take a look at the server configuration documents, Router/SMTP-Basics tab and check settings for "SMTP allowed within the local internet domain:" and
"Servers within the local Notes domain are reachable via SMTP over TCPIP:"

Wait - are you indicating the messages are formatted differently, depending on their method of delivery? i.e. via Notes, as opposed to via SMTP?

Yes. More accurately, the message format can be changed (converted) depending on the method of delivery.

This is the quote from Admin help, mail section, Supported routing, format, and access protocols  :

Domino transports and stores messages in both MIME format and Notes rich text format. The transit format of a message depends in part on the routing protocol used, and can differ from the format in which the message is stored in the destination mail file. When transferring messages over Notes routing the Router handles messages in either MIME or Notes format. Messages sent over SMTP are always sent in MIME format.
The format used to store a message depends on the storage preference specified in the user's Person document. A mail file can store messages in MIME format only, Notes rich text format only, or in both formats, accepting messages as is, regardless of format. Administrators should ensure that each user's Person document specifies the format preference appropriate to their mail client. For example, because IMAP clients require messages in MIME format, the Person document of a user who always accesses mail from an IMAP client should specify MIME as the format preference for incoming mail.
To ensure that users receive messages in the format best suited to their chosen mail clients, Domino converts messages between formats as needed. The Router may convert a message during transfer between servers or when delivering the message to a user's mail file. Conversion during transfer occurs when a message in Notes format must be sent over SMTP, or when routing a MIME message to a Notes user that is set to Notes Rich Text in their Person document. For example, The Domino IMAP and POP3 services also convert messages, as when an IMAP or POP3 client needs to retrieve a message stored in Notes format.
Because Notes routing can transport messages in MIME format, on networks that support both Notes routing and SMTP, a MIME message may travel over both protocols enroute to its destination.

Interesting. I have spent the weekend trying to verify that all servers are on the same Notes Named Network, and am attempting to get them to all transfer mail via the same protocol. My thought is to have them communicate via TCPIP, but to disable the SMTP task on each server hoping to force Notes Rich Text. Currently, I believe this will solve the problem of different users seeing different formatting on the same message. Do you think this is an appropriate way to get the desired result?

You might but don't have to disable SMTP server task (you might the task need it to accept mail from non-notes clients such as IMAP or POP3).  IMHO, it is less important then "SMTP allowed within the local internet domain:" setting. Check that it is disabled for all server, this way all mail between servers will be routed over NRPC .

Thanks for staying with me on this one.

Wouldn't it be more desirable to send all internal mail via SMTP? Wouldn't that increase the chances that mail sent outside the company would look as the users intended?

Thanks again for staying with me. I uncovered some Notes Named Network errors during the course of this, so hopefully my network is running smoother than before - PLUS, when I flame in RED text, everyone sees it!