I can't receive email in Mac Mail but I can send them

I use two email programmes on my Mac Mini - Thunderbird and Mac Mail. I can receive and send emails on Thudnerbird and I can send emails on Mac Mail but I can't receive the. Next to the mailbox is a squiggly line like a wave which I presume is the promblem but I don't know what that problem is.

I don't have this problem on my MacBook Air.

The Mac Mini is using Mac OS X Lion 10.7.5.

Thanks

roger