Databasemail, SQL server, stored procedures, IIS, plesk and mailenable

We run a jobs website. We are sending out newsletters to our subscribers. There are around 15000 subscribers. We have dedicated servers with no limits & we are not using smtp of web service provider. Most of the times when we send out the email, the system stops working & we are forced to restart the system. The sequence of events: 1. The website executes a stored procedure with search terms & email sent the the database 2. The stored procedure does the search and sends out email to databasemail. 3. Databasemail sends the emails to Mailenable (our SMTP server). I think plesk (which we use for website management) & IIS may also be active participant in this sequence. After couple of thousand emails are queued in databasemail, the server goes down. This happens most of the times, but at times the mail gets delivered and we get confirmation in the website. We are monitoring the server - it has 16GB RAM, hexacore processor & it says, even during the email time that only upto 40% is being used. I don't think it is a hardware issue. We are not sending emails all at once - stored proc populates databasemail table which is queued - databasemail sends mails to smtp server (mailenable) which again queques the emails and sends it at its own pace (about 10 to 15 minutes to send the mails out) Let us know if you will be able to resolve this issue?