Q: Lost emails (both ways)

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 so it's getting to be a big problem.

Anyone have an explanation ? Or better still a solution ?

Does it have something to do with iCloud?

I have a @mac.com email address (which I use) and I'd like to keep it. I also have a gmail.com address that I've tried to use (via mac mail) and there doesn't seem to be the same issues with it - but I have my @mac.com address on all of my business documents so I'd like it to work probably and to be able to rely on it.

Any ideas that might help me out? 

(Macbook pro OSX 10.8.5)