TD Bank Arbitrarily Refused Payment To My Vendors

You send 90K emails 2-3 times a week. You sound more like a SPAMer than a legitimate "small" business. While I don't know for sure, it may seem likely the "suspicious activity" isn't you. It is your vendor. If a bank gets numerous complaints about a specific vendor they may start to question that vendor. The fact that they are your email provider and you are sending basically a quarter of a million e-mails a week could raise a lot of red flags. In that the bank has complaints that this company is actually just a SPAM provider and is getting a lot of  chargeback requests so they will question people using them.

Another possible reason is the location of the company.  If they are not based in the US and are in a Country known for fraud, this could also be a reason for rejection.

As to the effect, if you are paying monthly and the payment was rejected. Did the email company notify you of the rejected payments? They are an email company...seems like that would be automatic. As you said this caused them not to email your subscribers for several days. Why didn't they notify you the second the payment was rejected? If you say they did then how come it caused a several day delay in the emails?

If these e-mails are sucn an income generator wny aren't you paying this company on a yearly or at least quarterly basis? Every legitimate provider I have dealt with makes the "monthly" payment option the most expensive option. They want to lock you in for a longer period. If your answer to that is you don't trust them or have some other "issue", I think you just answered your own question as to "why" these transactions are being blocked.

First, why is a regularly debited amount from a known vendor considered suspicious? (they say they can’t tell me why); and second, why am I not notified that such activity has caused an audit freezing activity on my card?
- NO bank will tell you the exact reason or notify you in advance. This is for many reasons, but mainly because they don't want to give the true criminals hints where they may adjust their actions to try and get around this.   While you don't appreciate this now, if you ever did have a fraudlent transaction that they blocked you would be very happy.