Emails and iCloud

My husband has been complaining for some time that his three devices do not work together (Mac Book Air, iPad Air and iPhone 6) and I have put it down to his luddite attitude to computers. HOWEVER - I am now experiencing the same thing.

My mail is set up in iCloud. Works perfectly on my iPhone 6 and iPad Air and Apple Watch. My Mac Mini is not showing SENT mail and today is not receiving mail.

I ended up sending an email to a customer four times because it wasn't showing up in sent. It didn't show up in sent on my other devices but if I SEND from the iPad/Phone it shows up on those devices but not on the Mac Mini. I have gone through several similar questions/answers and have checked what the results were. Have tried some and nothing works. Some time much later it showed up in my Recovered Mailbox and was tagged as junk.