Why does an email show up on my desktop IMac but not on my Iphone6 plus?

I could see even before I signed onto my Iphone I'd received an important email. I quickly signed on to read it; however it wasn't there. Where is it then? Why was a message showing up in my list of preview emails this had been sent.

Luckily I was home so i went to Imac & the email is there. It said on message preview with the Iphone it had been sent in the last 33 minutes, but as I said my email didn't show it. Same time showing when it was sent on Imac but the message was there.

This happens a lot when people use more than one device to sign into their email. Your main email program is set to 'download' the emails and delete them off the server. So, when you then go to your phone and sign in, the email appears gone. Go back to your main computer and you find the mail in your inbox.

Restart the devices