You have to always be so careful with sending out a large volume of e-mail, as the whole Spam issue is very complex.
This is the primary reason why I am so hesitant to place a newsletter from my website into action.
The main problem is unsolicited e-mails, that being sending random e-mails to people who have no connection to you or your site. However, you are looking at sending licenses for a product owned.
The big question is, to negate all potential problems, would it be better to simply place link on your website for owners to request the re-issued license, and manually e-mail out as requests come in?
Lunar Pages imposes a 600-800 e-mail per hour restriction on accounts. If your uses are in an address book within your e-mail program, perhaps it would be a good idea to e-mail them a few at a time. This would also mean that you could monitor bounce backs easily, and remove addresses from your address book.
You can send these mails, however, you must be sure not to send too much at once, I would consider sending no more than 200-400 messages an hour. Remember also that because of Lunar Pages zero tollerance for Spam, if we receive a number of complaints regarding Spam against your account we will have to suspend it.
It is very difficult to have a clear cut answer, simply because there isn't one. With 7500 users this must be a very popular product, however, I would advise perhaps finding a way to update license over the internet. Perhaps a routine similar to those found in AV software, which date stamps the software and warns you to upgrade your license from just under a year later. A system that could perhaps be adapted to suit your product.