Hello,

Head to http://www.senderbase.org. Use their interface to enter your server domain name or IP numbers and see if your server is listed anywhere as a spam server. Though you may not be a spammer, others who use the same server may be. Hence the advice of the chargrill is solid.

It's a shame that good things are so easily taken over by people who have ill intentions(Spammers). The rest of us end up jumping through hoops wasting time and money trying to accomplish what should be simplistic enough.

Another thought is to double check your email address format. The @ needs to be escaped (\@), especially if you are reading the list from a file.

Good Luck!