Applications :: Not Receiving All Of My Emails?

I'm a new convert to Mac, and I'm having a problem receiving all of my e-mail. I still have my PC desktop hooked up, and I am receiving messages there that I'm not receiving on my Mac. I am using Mail, version 3.3 on a MacBook Pro. My ISP is Comcast. I was using Outlook Express on my PC but converted to Outlook in order to export/import all of my messages, etc onto my Mac. That process seemed to work reasonably well.

Last week, when I set up my new computer, I got some messages but not others. By the time I left town for a conference, I stopped getting messages on my Mac altogether. When I got home, I saw that the messages were on my PC.

I spoke with Comcast and Mac yesterday about this problem. Comcast basically told me that they couldn't help me. With the Mac support person, I deleted my account and set it up again. That seemed to fix the problem, until later in the day when some messages were not delivered. There seems to be no pattern.

1. Yesterday, I received (on my Mac) two of the three daily messages I get from a list serv. This morning, I didn't receive the first message from the list serv. (But I did get it on my PC.)

2. Yesterday, I sent a message to someone and received a response on my Mac. I responded, but her response did not reach my Mac.

3. When signing up for these forums this morning, I received both automatic messages on my Mac.

4. Yesterday, I did not receive a response from someone that I e-mailed.

5. Yesterday, I received some messages from a google group but not others. There was no rhyme or reason to this either. I would receive one message from one person and not receive another from that same person (using the same e-mail account).

I am having no difficulty sending messages. And I believe that all of my messages are coming to my PC account. There are no messages on the server (aside from junk that Comcast filters for me), and no messages are landing in my junk folder on my Mac.