How to send thousands of promotional emails to opt in addresses without being blocked

IMHO sending emails to people who subscribed to a different startup is not okay. You could perhaps send a single "please opt in to our new venture" email to all those addresses, but not more than that.

You almost certainly want to look at mailing list handling services. Some examples (in random off the top of my head order):

• MailChimp
• Campaign Monitor
• AWeber
• Newsberry

All of the above should offer to send the emails from servers with well curated relationships with major ISPs, and proper technical settings for SPF records, reverse DNS and what else is needed to not get flagged as spam right away. And they do of course have unsubscription handling, i.e. a simple way to place an unsubscribe link in the footer of all emails and have unsubscriptions handled automatically.

I agree with the other answers, opt in is opt in for a specific purpose. You need to be very careful about what and how you send...it can be a very expensive mistake if you don't follow all CANN-SPAM regs.

But to answer your email question, there are hosted STMP relays that you can purchase access to such as SocketLabs and SMTP.com. I prefer these to a MailChimp if I'm going to be doing something proprietary and repetitively. I've had great success re-selling email marketing via Interspire's Email marketer combined with either SocketLabs or my own Co-Lo'ed server doing the routing.

Throttling (which is what you describe) is something my company uses to ensure the big three (AOL, Gmail, MSN/Hotmail) don't peg you as a spammer right off the bat...it could be 40% of your list, believe it or not. I would refrain from using it to "trick" the server hosts, as they'll eventually catch on and the result usually isn't very much fun.

While they may have been subscribers to "his last startup" - did they opt-in to receive e-mails from you? If they didn't, and/or they don't have a reasonable expectation that their e-mail addresses will be used for this new venture, it is spam. Just to get that out front.

As to your problem, if you aren't going to run your own server, you may want to look at one of the many third-party services out there for sending bulk e-mail. With the anti-spam endevours currently out there, there isn't much you can do via your ISP's mail service that won't eventually get tagged as spam. (regardless of if it is or isn't)