The syrupy Msgtag website gives the impression that this magical software will tell you every single time that one of your e-mails is opened. What peace of mind - what bliss! Problem is, it's not like that at all. Msgtag works on the same principle that spammers use, to see who's read their spam. All modern e-mail clients block it. This idea worked in 2003, but it usually doesn't work in 2007.
I naively paid for Msgtag Status. After 5 months of frustration I asked Msgtag for at least a partial refund. They refused. By all means try the free version and find out for yourself what it does, but don't make my mistake and pay for an up-market version of it.