Mail Inbox slow to receive mail

Hi, help please.
OS X Yosemite 10.10.2
Mail Version 8.2 (2070.6)
Plusnet
My incoming e-mails aren't arriving in the inbox until much later than they do on my iPad and Windows computer.
Clicking on "Get new messages in all accounts" doesn't make any difference.
They arrive eventually, maybe some hours later or the next day.
When I click "Mailboxes" there is an exclamation mark in a dark triangle see screenshot.
I haven't altered anything or messed with the mail settings so why is the mail late arriving?
Hope you you can offer some help to get the e-mail working properly again.

I had this problem once. One of two things fixed it and I cannot remember which. The first one is to try going into the mailbox menu and at the bottom is rebuild. The second option is to delete your mail file (Library/Mail) and then re-download all of your messages again. The second option only works though if all of your mail is stored online and can be re-downloaded. This problem kept happening to me though so I wound up switching to Airmail.

Hello danipoak,
Thanks for the reply.
Tried the rebuild but still the same.
These two screenshots show what I'm getting when i click on the exclamation mark now.
The spinning circle for Plusnet email POP just keeps on spinning.

I haven't received any mail at all this morning although it's arriving on the iPad OK.
Strange this problem has started because as i said I've not touched any settings
Anyone got any ideas how to fix it.

UPDATE.
The incoming mail problem is now sorted.
Thanks to Plusnet's very helpful tech support.

I know it's a bit late, but could post the solution you found?

Hello damanieu,
I had to ask Plusnet tech guys for help in getting the problem sorted out.
As far as I can remember I think they talked me through some settings.
Sorry I can't be any more help