Here are Arial we get many instances where clients send email perfectly for long time and then bamm! things start falling apart. Many times a software is, like ours, is blamed for the issues because it is a logical thing to look at first. The truth of the matter is that Campaign Enterprise, Email Marketing Director, or any other emailing software does not change...but the circumstances around it do. Sometimes a dreaded OS update happens overnight and introduce new "support files" that your emailing software requires that causes a difference in functionality than before. You might have a corruption of some of the email software files, but this is rare.
The #1 culprit of these "it just started happening" scenarios is Blacklisting. The internet is a dynamic place. New technologies to protect the general public from SPAMers, Phishers and the like come online unknowingly and what was working great before all of a sudden just falls apart. Email distribution management is not a noun but a verb. It is an active thing that requires monitoring and maintenance. Part of your maintenance schedule to make sure everything is working well should include a scan to see if your SMTP server system is on any blacklists. We recommend using a website called http://mxtoolbox.com. With using this website, you can find out in an instant if you are being blacklisted by many blacklist sites. It only take a few problems with your SMTP server or a few complaints from cranky recipients to be blacklisted so check it at least weekly or before you send out any major email blasts.
As email distribution professionals, we need to play by the rules and be good internet citizens even if the rules changes. Remember, it is not IF the rules change, it is WHEN, so stay diligent.
Also, with visiting a free analysis tool like MXToolBox, be sure to visit a few of their advertiser and support their effort.