Do your research!!!

Email can have delivery issues for a number of reasons. Actually, I'm afraid that there are many reasons mail could bounce. In fact, there are so many ways it could fail that sometimes I'm amazed that it works at all. We have a department in this company filled with employees that do nothing but work on email delivery. After hearing all the variables that go into a successful mailing I am always trying to shift a data purchase into a data rental. Unless you have your own IT staff that is handling all of the small details pertaining to a successful mailing list, you could come across many problems.

For starters, each and every ISP has different email caps. If you exceed the cap you will bounce the emails going to that ISP. It took us three years to get all of the IP cap settings. Once we setup all of the cap setting in our mailing software it eliminated 30% of the bounces we were receiving.
The "From" address might not match an account on the email server.
The ISP might require that email comes via a connection (dialup or DSL) actually provided by the ISP - sending using someone else's connection might not be allowed.
The ISP might require you to authenticate before sending email and you haven't. Our COO created a entirely new position in the industry. He hired an executive to verify email. That is all he does all day long. When a company sends email to a list (especially b2b lists) the server that sent the email will receive "verification/authentication emails" that requires a human to verify that you are a real live human being and not a bot or a spam source. This is usually caused by the software protecting the end-user. Approximately 30% of the email that we deploy needs verification. Rather than loosing all of the valuable email contacts we have an executive in place to perform this repetitious task.
A mail server somewhere could be misconfigured.
Chances are your email client is misconfigured. Double check out outgoing or "SMTP" server settings, and double check with your ISP to ensure that you have them set correctly.
Some bounces will occur for reasons of "resources being temporarily unavailable." or "Out of memory" which typically indicate a problem with a mail server that you probably don't have any control over. They are, in general, temporary, and should resolve themselves over time.
You also have to be very careful with content filters. Much like blacklists, content filters are an approach many ISPs now implement to stem the tide of spam for their clients. Most will simply discard email that looks like spam, but some servers will actually send a bounce. Phrases in the bounce message like "Message looks like spam", "keywords rejected by the antispam content filter", "scored too high on spam scale" and similar means that your email, for whatever reason, tripped the spam filters on the receiving end. Your email looks too much like spam. We have to send all of our mail advertisements through our proprietary optimization software. Our unique software strings thousands of algorithm check points and I also believe that our algorithm software stands alone in the fact that it is predictive. Our predictive algorithm software self-updates every month keeping up the forever changing algorithms. Each ISP has different algorithms that will hinder a successful mailing. What kind of optimization software are you using?
There is another form of ISP caps but unlike the ISP caps that bounce too many messages going to AOL or Yahoo it will begin to bounce when too many messages are coming from the same IP. For this reason our mailing software rotates a string of 20 IP’s, even for small volume mailings. How many IP’s are you rotating within your mailing software?
Regardless of the amount of IP’s you are using they all must be clean. If you have one IP that is blacklisted you are going to have major problems with deliverability.
You need to make sure you have a reverse DNS setup or 80% of the email you send will bounce. You can find out if your dns is setup correctly at the following site. www.dnsstuff.com You can also check the status of your ip’s there.
As if all of this was not enough Microsoft based email servers such as hotmail require you to be a bonded sender in order to send any kind of volume to their email addresses. Hotmail has implemented a third party certification solution which gives senders another option to increase their ability to reach Hotmail users. The Bonded Sender Program uses sender identification and reputation to establish the “legitimacy” of a sender. Mail sent from senders that are enrolled in the Bonded Sender program skip much of the Hotmail anti-spam filters. If you are not a bonded sender you might as well just scrub all of the hotmail email out of your list. Unfortunately the fees to become a bonded sender are hefty and you must pay these fees each and every year. Are you a bonded sender?
Similar to a bonded sending program or feedback loops it is extremely important to get whitelisted with all of the major ISP’s. The process takes quite a while and you are not automatically accepted by all ISP’s. Which ISP’s are you whitelisted with?
Sometimes a bounce has nothing at all to do with you. In some cases your recipient has too much email and their server isn't accepting any more. This is most common with web-based email services like Hotmail or Yahoo, which have limits on how much mail you can accumulate.
The other major problem with mailing nowadays is the domain keys you are using. What domain keys are you using? Are you using SPF or Sender-ID or Goodmail DK or DKIM or all of the previous or others?
Another major reason companies have problems with delivery is the fact that they are not on feedback loops. Once we got ourselves on all the feedback loops our delivery dramatically increased. What feedback loops are you on?

To get a better understanding of why email is having delivery issues please answer the following questions.

What are your cap settings for the following ISP’s, hotmail, yahoo, aol, msn, Comcast, version, roadrunner, cox, bellsouth, juno, sbcglobal and rocketmail?
What are the domain keys you are using for the same ISP’s?
Do you have somebody going through the authentication process?
Did you double check the configuration of your mail server?
Are any of your IP’s blacklisted?
Are any of your IP’s whitelisted and if so how many?
Are you using software to remove the algorithms in your ads? What kind of software?
How many IP’s are you rotating within your mailing software?
Do you have a reverse dns setup?
Do you have any configuration errors on DNSSTUFF?
Are you a bonded sender?
What feedback loops are you on?