Just an input from a web host perspective.

Yes, most web hosts limit the access to their resources for sending emails.

As a web host we allow 200 emails per hour and per domain. If you send more emails they will bounce back. Now each host have different limits so make sure you ask your host, it will save you some time.

Now your host my also have a limit on the total of emails you can send in one mailing list, some limit it to 5000, 6000 or so. This means that even if you throttle your email list, when you reach the max, it's over.

In this thread I saw this comment :

Most hosts will limit your sending capacities for email and they don't tell you unless you ask so you think everything worked fine and find out later only a couple of hundred went out
I feel a little bit of sadness and disappointment in this comment but it's not a secret so just ask your host.

Why web hosts are doing that ? first to avoid having spammers on their servers and then to protect their customers against server abuse.

As a web store owner, you goal is probably to get orders and make money. Imagine if your zencart store is on a server where 10 people or more try to send a newsletter to their thousands of customers at the same time. Of course while they are doing that, one of your customer is clicking on the place order button to process his credit card.

What happens then is that the credit card transaction is probably not gonna go through and time out because the server resources are abused by mailing list senders (or other abusers).

Web hosts can't make everybody happy, their often prefer to make sure that the server resources cannot be abused to make sure that most of their customers are gonna be happy. Even if it makes a few unhappy.

Be careful also, most hosts will suspend an account that abuses the server too often.