Emails getting stuck in outbox

I have had a chronic issue with emails getting stuck in the outbox. Some have attachments, some don't. I have this issue with numerous email addresses - not just one. And not all emails to a given recipient get stuck. This happens on both my phone (2 versions) and macbook pro and over numerous system versions. I've checked the connection doctor and internet connection. Seems to me that after forwarding several emails to a different email of mine, a few of them "dislodged" and were sent out - but I can't trace that back.

This has caused several uncomfortable situations between me and clients and I'm determined to get to the bottom of it.

Thanks in advance for any suggestions/ideas?

- Sara