Linux Mail Server Setup and Howto Guide

We are using sendmail on CentOs 5 to distribute our e-mail.

We are sending them out with a program called ActiveCampaign. This program reports that pretty much all mails have been sent out. The sendmail queue shows about 2% of the mails still in the queue 24 hours after having started to send. So still 98% of the mails do go out.

However, from the low response rate it seemed that much less mails actually get sent. So we contacted the companies SenderScore and ReturnPath to get their view. They told us that we have more than 10% wrong user names in our list, but also that we have a very low acceptance rate, i.e. the rate of how many e-mails actually land in the inbox of the desired recipient (35 – 50% apparently), partially due to "an infrastructure problem". They also said there seems to be a long delay on mails arriving – they had us do a test with 1000 addresses and it took apparently half an hour until the last mail arrived at their end.

As info, in general, we are sending more than 80,000 emails per week (we are not spamming, these are all existing customers).

And they want $20,000 to give us the details so we can solve the problem. Apparently I need to sign a contract where I am going to have to pay $20,000 a year so they will monitor our email traffic.

I should add that we are using DKIM and have added the various things required by gmail and Yahoo for e-mail to be accepted. And single test mails to any of these various mail servers do seem to arrive without problems within seconds.

My question is – can I solve this problem another way? $20,000 is out of proportion and unviable for us. It is impossible that everyone needs to monitor his e-mail at a cost of $20,000 a year. So how are other people solving this problem?

What can be the cause for something like that?

Is there a way for us to track and trace problems to find the cause (without the $20,000)?

Any programs / tools / utilities one can use?