Not sure I completely understand, but it's possible to have an IMAP account in TB that does not keep message bodies on the local computer, only the headers. The setting is in Tools/Account Settings, Synchronization & Storage, Message Synchronizing, uncheck 'Keep messages in all folders for this account on this computer'.

And that won't take up a lot of space on the server? We only have 100 GB and 50ish users to get on the server. One of them has 10 GB in their email. And we know the users are not going to clean up their emails. Basically we just need a way for Thunderbird to be able to send emails from the server without it downloading what is on the Outlook Server ... but still syncing what is sent to Outlook. Does that make more sense? I apologize for the confusion.

When messages are kept on the computer, as usual, the mail folder consists of an mbox file (large, no file extension), and an index file of the same name and .msf extension. When messages are not kept, there is no mbox file, so you can judge the space needed by looking at the msf file, typically about 1KB per message. Open the profile folder from Help/Troubleshooting Information, click Open Folder, look in the ImapMail subfolders.

I thought you might be able to just add another smtp server and have the existing accounts use it for sending, without adding the incoming server. But I don't think you could have the sent mail sync with the Sent folder seen by the Outlook users.

Okay, I think we have a solution in place. We're going to test with a few local people for the next week or so to ensure this is doing what we want it to do and then we'll implement company wide.

Thanks for your help!