I think it's not that difficult to roll something like that on your own. It would take a day or so to build. But you should be much better off using something existing.
Still, some things I found important when doing Bulk-Email:
Don't use your own SMTP to send those.
Lookup the MX entries for your recipients and connect the SMTPClient to their SMTP, this gives you the benefit that you get an instant error msg back when the mailbox is not found or something else is wrong, and you don't stress your own server.
There was also a little bug in SmtpClient that gave me trouble because the hostname isn't a fully qualified domain name (as requested by the SMTP spec) but the machine name (and that's an internal field inside SmtpClient). Some SMTP servers take that seriously and want a domainname instead, so I ended up setting that field with reflection to a FQDN. (Hopefully this was changed since)
