Capturing email messages is one of the most common things people do with the Pro editions of DEVONthink. (Sorry, but neither DEVONnote or DEVONthink Personal support indexing emails, though the Personal edition does allow you to import them via drag and drop.)
One of the differences between the editions is how you can search for emails. In DEVONthink Pro, the emails aren’t indexed but you can find them by searching by Name (subject line) or URL (sender). In DEVONthink Pro Office, the subject line, header, and the body content is indexed.
If you switch over to DEVONthink Pro Office at some point, the already imported email won’t be indexed, only newly imported messages. So do you have to try and reimport all those emails? Of course not. Do a Tools > Rebuild Database… and when DEVONthink rebuilds the index for the database, it will index the contents of the previously imported messages, too.