If you are experiencing email delivery problems or delay issues when sending to the larger ISP’s then most likely there is a reputation problem with your IP address.  The first thing any ISP will tell you when you ask how you can improve your reputation is to follow their best practices guide.  The problem is that every ISP has its own “Best Practices” guide and this can get overwhelming.  Below is a list of the top items that are common between the Major ISP’s (AOL, MSN, and Hotmail.)

1.       Adhere to the CAN-SPAM Act – Senders should read the CAN-SPAM Act and ensure that their e-mail adheres to the requirements: http://business.ftc.gov/documents/bus61-can-spam-act-compliance-guide-business

2.       Clean your address list – After receiving a permanent non-delivery response with an error code between 500 and 599 you should remove the sender from your list.

3.       Secure your Mail Server – Having an open relay will get your IP address added to the major blacklists and cripple your sending.
 http://en.wikipedia.org/wiki/Open_mail_relay

4.       Branding – Use a consistent from header and include your company name in the Subject line. This will not only allow your recipients to quickly know who the message is from, it also allows them to add your address to their personal white list.   Recipients who don’t know where the message came from are more inclined to hit the SPAM button, which is going to hurt your reputation with that ISP.

5.       Authentication – If you enable domain keys, DKIM, and SPF it will cover you with major ISP’s that use authentication.

6.       Easy Unsubscribe method –  If the recipient does not know how to unsubscribe or does so and still gets another email message you can guarantee that they are going to hit the SPAM button.

7.       Use Opt-In Addresses only – Ensure that you are only sending mail to users who requested it. It is not advisable to purchase mailing lists or subscribe users by having an opt-in checkbox automatically checked on your website.

This is by no means the end all list of best practices but if you don’t follow the above items at a minimum you are guaranteed to encounter some type of reputation problem.