SQL Server 2014 dbmail won't send attachment over 15MB, file size limit set to 60 MB

Question

I have an automated e-mail that goes out and will sometimes have an attachment, this has been running for over 3-4 years now with no issue, until now.

Up to now, attachments were maxing out at just over 14MB, but the other day we had two that failed to send. One was 16MB and the other was 17MB. The Maximum File Size parameter for Database Mail in our SQL Server was set to 60MB. Checked Exchange and SMTP settings and they are both set to allow unlimited or up to 100MB attachments.

What could be the issue? When I kick the process to send an email with the 14 MB file, it queues up within a second. When I try with the 16MB file, it spins and finally times out after a minute.

The files are PDF's so nothing restrictive on that front. Tried increasing the Maximum File Size parameter to 100MB, but that made no difference. Exchange logs don't show anything, so I don't think it's ever even getting to that point. It has to be something in SQL Server, but I have run out of places to look.