Client sends email newsletter to 5000+ recepients

you probably going to have to use PHP. here is a site that has a vouple suggestions on using PHP to auto generate e-mails.  http://www.velocityreviews.com/forums/t159057-auto-generate-email-w-attachment.html

I forgot to add i know it says attachments but you can strip that out.

The normal solution is to use an email marketing provider.  The reason being is that they have their MTA's setup to deliver large quantities of email on their IP addresses for which they have agreements and monitoring in place to ensure that their IP addresses are not blocked and your mail will go through.  Plus your mail will go through much more quickly and reliably as they are not using normal smtp servers like exchange or sendmail, they are using specialty MTA's.

If you do decide to do this yourself DO NOT USE PHP.  The delivery handling of greylisting, deferments, and other issues that arise when you send bulk email will not be handled.  Plus PHP will not be multithreaded and will take forever to delivery those emails and will most likely get clogged and die.

But if you really want to do this in house then you should use one of the commercial MTA's that most of the big email markeing providers use.  The most pouplar are:

Port25 - http://www.port25.com/
StrongMail - http://www.strongmail.com/

Most importantly, DO NOT SEND these from any IP address that you are sending regular mail on.  You will be blacklisted (no matter what your content is) and you will have to monitor your ip reputation to remove blocks at verizon, comcast, etc.  You will also get deferred at yahoo, hotmail and aol.  Keep them separate and don't let your bulk mail affect your normal everyday mail.  And if you want those 5k to go through quick, you're gonna need multiple ips.

So do you guys have any experience with those email marketing providers?  Which ones are more reputable and which ones would you recommend?  
Thank you!!