Well, you should be looking to stay under 300 per hour -- 400 is the absolute stop point, at which your account is suspended, and that counts incoming mail too (plus any random outgoing mail for customer order confirmations, etc.)! So, you're looking to take 3 hours to send your current list.

If Zencart doesn't have any way to throttle email sending (and being based on osCommerce, it may not have), you have at least two choices.

1) Export your address list (possibly by cut and paste) to a mailer program such as PHPlist, which can be set to obey rate caps.

2) Modify the code to introduce a delay after each send. I've suggested this to osCommerce users before, but I don't know if anyone actually tried implementing it. The idea is to find every point where mail is actually sent out (e.g., the PHP mail() function) and slip in a sleep(12); after it. Maybe you could have a global switch to use the sleep() only during bulk mailings (newsletter) and leave it off other mailings (customer orders, etc.).

When did the 800 to 400 cut happen?    I support a nonprofit Youth Center organization and I've been sending news letters successfully until tonight.   I tried to send out 560 emails at 10 every 10 seconds... which appears to blow through any limits since that's 50/minute.    

* - edited, we do not allow this type of comment via the forums