I was tempted to chime in on David's post down below as I'm kind of having the same problem. Maybe. After changing my Mac Mail from POP to IMAP, it fails to read data from the server except on start-up or if I Take All Accounts Offline, then Take All Accounts Online. I've spent several unfruitful hours with Telus support (Canada) trying to help me resolve this. They say the settings are correct and there must be something wrong with Mac Mail. But I can't find anything online to steer me towards fixing this mess, including on Apple's website.
So basically it works Sending, but doesn't retrieve new emails for me unless I go Offline/Online with the account and hit Get Mail. A total annoyance.
Yes, with Telus. 3 different Telus agents tried to help me, even coming on to my computer remotely, and they all said it's a problem with Mac Mail and one of it's advanced settings and to contact Apple. They said SSL should be deselected. So obviously I've tried every foreseeable scenario and last night I selected SSL. It connected to port 993 and for 10 minutes was working perfectly, automatically and instantly receiving my test messages. Then on the fourth test (this is how suspicious I am), it wouldn't retrieve and I got the exclamation symbol. Today, SSL selected or not, it will not retrieve my email without my ridiculous work-around (mentioned above).
I've read that Telus blocks some port access. Might be worth your while to get a gmail account and have it automatically check your Telus account. I never recommend anyone use their ISP's email systems as you tend to be held hostage by the ISP if you ever want to change providers.
John Davidson | President / Creative Director | Magic Feather Inc.