Hi,
I had problems when sending mails to multiple recipients using some SMTP servers. After comparing the headers produced by AIM and Thunderbird I recognized, that Thunderbird uses "," as delemiter between multiple addresses and AIM using ";". According to rfc6068 the ","seems to be the correct way. After changing the 3 occurences in the SendMail() function it worked fine with all my test SMPT providers. Maybe you can double check if "," realy is correct and if so update your code base.
best regards, Christian