I have a dedicated windows server with runs iis 6.0 as smtp server and i use .net to send emails. I run a blogging website, it has a comment section, when anyone leaves a comment i want to send emails to whoever left a comment on that blog. Some blogs have over 100 suscribers.
I been sending them email in a loop by using "to" field for every subscriber.
My question is, whats the best way of doing it? I dont want to be flaged as spam nor overload the mail server.
With 100 or even 10,000 subscribers, you will not overload the email server. Sending a separate email with the recipient in the To: field is the way to go. If you include multiple recipients as To: or Cc:, they will see each other's email address which violates privacy. If you include multiple recipients in Bcc: that is a strong indicator to an anti-spam system that the incoming email may not be legitimate.
Not being flagged as spam is a complex subject.
The first, most important rule is to make sure that the people receiving the email know it is an email they signed up for. Being clear in the subject line goes a long way, e.g.:
Your comment on MyBlog.com
Beyond that, some things to consider include:
An alternative is to use a service like Amazon's Simple Email Service. They provide a robust infrastructure and manage many key aspects of deliverability for you. They provide a free usage tier up to 2000 emails/day.
UPDATE
Regarding whether or not bcc can trigger spam, here's a selection of the google results from does bcc trigger spam
If you put lots and lots of names in the Bcc field, the email may be marked as spam.
http://www.airsquare.com/support/email-marketing/prevent-your-emails-going-to-spam.cfm
It is better to send an individual message to each recipient, rather than using multiple addresses in the BCC field because many spam filters (and many ISP's) automatically flag multiple recipients as spam.
http://www.xequte.com/support/maillistking/spamissues.html
Putting multiple addressees in the BCC field was a trick used early by spammers so many spam filters are set up to recognize emails with multiple BCC recipients as spam.