Some emails not coming through

For some time now I have had the frequent problem of messages not showing up in the "sent messages" box. It's irritating not to be able to have a copy of all the emails I send, but I can live with it.

For the past couple of weeks however, I've noticed that some of my emails are not being delivered (even if they appear in the "sent messages" box!!) and I also don't receive all the emails people send to me.

I've already lost two contracts this way in the past weeks (and am getting very paranoid whenever I send an email or wait for an answer. )

It's getting to be a big problem.

Anyone have an explanation ? Or better still a solution ?

I'd really like to keep my mac.com email address (there seems not to be a problem with my gmail.com address via mac mail).

I'm on a macbook pro OSX 10.8.5 & mail via iCloud