Don't escape the @ in a single quoted string. You should either use:

or

For debugging things like this, I'd sugest having the program write out the mail being generated to a file, rather than pushing it to sendmail, and taking a look at it. If you want everything in the 'To' line, use the format:

I thought e-mail addresses format was explained in RFC822/RFC2822, or RFC821/RFC2821, but I'm not seeing it.

Update: the address specs were in RFC822 and RFC2822