Sending mails with huge attachments can lead to problems. From the client side (e.g. Thunderbird) all seems to be fine but actually the mail message is never send and the user simple does get any information about this failure.
If the mail size exceeds the maximum allowed size (e.g. 5MB), davmail simply ignores the error message produced by OWA (from the log: "HTTP/1.1 507 Insufficient Storage[\r][\n]").
I attached the part of the log file (mail content removed).