In my case I ended up creating an entirely new mail file to resolve the issue in my specific situation. Others waited for a hotfix from IBM which seemed to resolve the issue.
However, we now have a formal fix – Domino 8.5 Interim Fix 2, which includes:
In Lotus Domino 8.5, you will see the IMAP task crash your Domino Server in certain instances. This issue has been reported to Quality Engineering as SPR #JCHS7NNM56. A fix is available for this issue.
We have seen the IMAP task crash in certain instances. So far we have only seen and reproduced this crash with RFC822 attachments embedded in the messages body field.
This issue is currently addressed under SPR #JCHS7NNM56.
The fix forces embedded RFC822 blobs in the MIME body to have properly constructed root entities to avoid IMAP crashes.
Excellent news… Get Interim Fix 2 from Fix Central (or IBM Support) today.
[Thanks to Liam Harpur for the tip]