SmtpClient send 1000 emails - solutions?

On my website I offer users to send email to other registered users.
Some times they need to send to up to 1000 users. As there are so many emails I have heard that not all email is sent and some blocked.  
Is there a way to send so many email without being blocked? Is Microsoft, Google or other companies offering solutions for this that you know of?  My website is in .net. I am using MailMessage. I send the email to SmtpClient.
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I think more information is needed.  Who's blocking you and is it the same email server?

Maybe because your email server is not correctly configured (no rDNS, etc.) and that's why some email servers are flagging your emails as spam.

There is no ONE solution that will fix your problem.  You just have to ensure that you have Reverse DNS, Valid Hostname, SMTP Banner are configured correctly and that your SMTP IP address as your domain name are not blacklisted.  If your SMTP ip address was used by a spammer before then it's probably registered in Spam Database and Blacklist Check.  I don't think this is the case because most of your emails are getting through fine as you said.

Then even after all that's checked, there is no guarantee because for example, if you're sending 100 emails in a span of 1 minute to 100 recipients in the same domain you may get flagged as a spammer, even when it's legitimate.

If everything is properly configured and your email are still getting caught as spam then it's the job of the recipient to notify their email admin to put your SMTP server IP address and domain name in the whitelist as a safe sender.
Dave BaldwinFixer of ProblemsCommented:
Many ISPs limit the number of emails that you can send in a time period like each hour or each day.  100 to 250 are typical limits.  Web hosts also have similar limits.

In addition, you can't control what get put in the spam folders, you can just limit it by checking with the sites that rate email for spam content.

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