Mail not sending -- Mail

Hi,
I've switched over completely to my .mac account and am using Mail exclusively from OSX's Mail program. The past few weeks, I've found out that a few of my e-mails from my .mac address aren't getting to their designated recipients.

My wife, who uses a Hotmail address, and my banker who uses an address within the bank system. I've sent him 3 e-mails this week and only 1 has gotten through?? Would this be a Mail problem, or an ISP problem??? It never happened when I used the e-mail from the ISP provider.

Thanks.

Frank