You are running a email newsletter with a large number of subscribers or are running a email marketing service that supposes users sending emails to each others.
Your first problem is mailers configuration to deliver maximum emails per second.
Another problem is email delivery, if you send many emails your service is watched for sure.
To maximize your email delivery you should follow some steps
1) Double opt-in members on registration. Your members should fill in a form and submit it, by checking your Terms of Service – this is the first step of opt-in. As a second opt-in step they should confirm their email address and activate their membership by clicking a activation url.
2) Get maximum information from members. Some list owners get only name and email address on users registration, but it is far not enough. We’d suggest you get more data including first name, last name, email address, mailing address, phone, birth date, member IP, registration date.
3) Insert correct email footers. Insert into each email a footer with member name, email address, subscription date, subscription IP address, unsubscribe link.
4) One click unsubscribe. Make sure all your members may unsubscribe by clicking unsubscribe link in your email footers and maximum clicking a unsubscribe decision confirmation button. In case you ask your users enter a password or any other earlier saved data that confirm user identity, they may easily click spam button.
5) Make sure your from email address is a real email address. If your email address is not a real one email hosters will detect it and may send your emails to spam folder.
6) Check your email inbox for bounced emails, autoresponder emails, unsubscribes. Set-up a automatic tool that will check your from email inbox for non delivered emails. Hosters don’t like if you try to send emails to non-existing email addresses, you better don’t do it. If you send emails to autoresponders you may achieve to ping-pong effect when you send a email, get a autoresponder answer then resend the email and so on. At last you end up with your server down and email address marked as spam sender.
If any member of yours sends you a email with subject unsubscribe such a member should be automatically unsubscribed, otherwise your email address will be marked as spam mailer.
7) Set correct Reverse DNS records. Make sure you have setup reverse DNS records. Without reverse DNS your emails most probably won’t be delivered.
8) Add SPF DNS records. Setup correct SPF records for your domain or email address.
9) Add DKIM records. Setup correct DKIM records for your domain.
10) Add Domain Keys records. Setup correct Domain Keys records for your domain.
11) ‘List-Unsubscribe’ header. Add to your email headers a List-Unsubscribe email address or a URL. The idea is the following one – when member clicks mark as spam button, the unsubscribe link is clicked or a unsubscribe email is sent to your email address. In this case, member spam complaint is not considered as spam as member is automatically unsubscribed from your mailing lists. This header works for gmail only.
12) ‘Precedence: bulk’ header. A new header is required by gmail in order to indicate you run a bulk mailing service.
For more details on Gmail, Yahoo and others email delivery read the guides below