IMAP email account receiving messages with attachments of 25MB or greater can take hours to be processed.
Steps to Reproduce
Set up an IMAP account on your instance.
Send email messages to the IMAP account with attachments of 25MB or more.
Wait for the instance to download the messages.
Note that the email reader takes 2-5 hours to process the email.
If large attachments are required, use POP3 connections rather than IMAP to avoid this issue.
Related Problem: PRB742904