This just started about six weeks ago.
Incomming mail arrives fine.
Outgoing mail is delivered fine, unless it contains a PDF attachment, and only if that PDF attachment is 2MB+ in size. Should this 2MB attachment be split into two 1MB attachments, and sent one at a time, both arrive fine.
There are no send or receive limits set for Exchange.
These 2MB+ PDF messages just sit in the queue. We first noticed this after 5 days had passed.
After a reboot, they are sent out and delivered as intended. For a period everything works fine, then some random time later (few days to a week, no definate trigger,) process starts over again. 2MB+ of other files can get sent fine, PDFs 2MB+ sit in queue. Reboot, delivered, rinse and repeat. No significant errors being reported with OS, Exchange, DNS, AV.