I would bet the "other company" is the issue and may have you on a spam or black list. If the ISP is not willing to talk to them about it, you're hooped.
Others here have suggested simple tests to determine the source of the issue but what it comes down to is you're exceeding a typical users emailing so the issue will come around again.
Long term, you're going to want to either (as per other suggestions):
a) migrate to a newsletter service like MailChimp. (first 2,000 subscribers free but limits on total emails x email addresses until you pay) Take a look at their pricing for an idea of what it would cost you. MadMimi starts with 2500 and has cheaper tiers.
b) get your own domain and manage your email list there. Then its your new host providers limitations you're dealing with. Some present unlimited but none want to be spammers so read the fine print. "unlimited" rarely is. You don't want to just migrate your problem.
You could also try breaking up your list with multiple sends. That may set off the alarms less but the problem will still come up. If you show signs of spamming, the bots will shut you down automatically.
I'm not going to get too involved with the discussion, but would only like to ask why you seem to be running several antivirus & antimalware programs at the same time? Generally this is felt not to be a good practice.
Originally Posted by deanvet
And, too add another possible complication, you state that you are using XP-pro a pretty old OS that was even less capable of handling multi antivirus programs running together? I'm surprised that no one else has commented on this, even though it probably doesn't have much to do with your email suspension.
If you want to see if you're on a blacklist, go to whatismyip.com, note your IP address showing, and then select Blacklist Check from the top menu. Enter your IP in the box if it is not already there and click the Check Blacklists button. In a few seconds you'll see if you're on any of these blacklist sites. Interestingly enough I'm showing on two sites ... although I don't have any problems sending emails. Apparently however some spammer(s) had recently been spoofing my email address as I was getting a lot of bounce backs on spam messages that hadn't been sent from me. That probably caused these listings.
About a year or so ago, we had an incident at work where we were not able to send outgoing email and when I contacted our ISP, I was told that our ISP wasn't blocking our outgoing email but that we were on a black list apparently resulting from some malware or something that actually had sent spam from our network. When I attempted to track down the offending computer on our network, to my chagrin I found it was me! One of the blacklist sites (forgotten which) allowed me to view the offending email and turns out I had infected my computer by opening an email attachment but in my defense it did appear to be a legitimate attachment from a well known freight company. I'm normally careful with email attachments but this was a good one that was hard to spot. Anyway, antivirus/malware programs didn't catch it and eventually had to roll back my computer to a date before I'd received the email to get rid of it. The ISP advised I had to notify the various blacklist sites that the problem was resolved to get off their blacklists. Some would drop us off the list immediately (with the warning that we would be back on if the problem persisted) and others took a few days of no spam activity before we were dropped off. It took nearly a week before the last blacklist dropped us. Fortunately our email service was back up before that time.
Anyway this story is probably irrelevant to your situation other than to check the black list sites but is a good warning to others about how much trouble one little mistake can cause. My little "indiscretion" with that attachment had shut down outgoing email for our whole company for a couple of days!
OP last posted on the 17th and still has people posting "aDviCe" 3 days later, for once I wasn't one of them..