Without wishing to rain on a parade: server-oriented issues such as these may be
fiendishly difficult to 'fix' for broad ranges of users. Two users of the same e-mail service, both using Macs in one room, might find different results.
The likelihood of making this type of problem
reproducible may be increased by:
- frequently saving a draft with multiple large attachments and clicking Send before the e-mail client has completed the most recent save
– it may help to visualise things overlapping and finally competing.
First: reproducible, if you can (without too much difficulty).
Then: calm, careful attention to detail with whatever Apple may request for diagnosis purposes.