Any word on this? It appears to be a smartermail issue.
Outlook 2013, Outlook 2016, And EMClient all have the same issue with sending attachments >20MB. Suspect phone mail clients are also effected, I just haven't been able to find a client that allows me to attach more then 20MB worth of attachments.
I've ensured that attachment sizes are all high. And can send via SMTP and webmail with no issue.