I cannot send email with photo attachments (example 127kb). No restriction when I compose, attach, and then click send. The email just doesn’t ever show up as sent (like a regualr email) and is not ever in the outbox. It is never seen by recipient. I use hotmail and when I check in my Outlook site account it doesn’t appear there either. Photo attachments come into my eM Client okay from outside. At one time, emails with attachments sent using eM Client worked fine using same laptop, location and Wifi. Sorry I do not see any resolution to similar issues.
If you right-click on Smart Folders (top left of your folder list) and choose Outbox, do you see the missing messages in the Smart Folders Outbox?