When importing email, Apple Mail is going to provide the best performance as it supports native plugins. Other email applications require the use of Applescript to interact with DEVONthink and we can’t guarantee the performance, especially with large mailboxes. But if your email client supports an export to the UNIX .mbox format you can drag the exported .mbox file to the DEVONthink Pro Office dock icon to scan it for messages.
Secret tip: After Apple Mail, Microsoft Outlook is one of the more common email clients. Unfortunately it doesn’t appear to support exporting to the .mbox format. However, if you grab a folder or a mailbox and drag it to your Desktop, it automatically exports an .mbox file!
Do note that the more messages you have to process, the longer it takes DEVONthink to process and import them. So if you have multiple mailboxes, drag them individually. You’ll have more .mbox files to import but the imports will be faster. And lastly and sadly, this does not work with smart folders.