Trying to send zip file of about 43 KB. It won’t go through and stays in Outbox.
EM Client has no restriction with attachment size, the restriction only applies to the mail server. However, eM has very little tolerant against unstable internet (latency) connection when its comes to sending mail. At least that the case with Google service in my experience.
Try another SMTP server or check your internet connection against latency.
If the problem persists, send us your SMTP logs. Thank you.