Emails are being rejected by some email providers

I have this asp.net application that sends out emails. The SMTP server is iis6. The emails are being sent on behalf of our application users and they have different domain names. Most of the email providers, like gmail and yahoo, would accept the emails but some of them not e.g. AOL.

This is the message that users receive from our SMTP server after a few minutes:

Subject: Delivery Status Notification (Failure) This is an automatically generated Delivery Status Notification. Unable to deliver message to the following recipients, due to being unable to connect successfully to the destination mail server.

Solution 1:

The message you posted is a failure to connect which doesn't necessarily mean they refused your email, it seems more likely a dns issue or some other connection issue. Some things to check/think about are:

Can you check your application server to make sure it is properly resolving the dns and MX records for the domain you are trying to email.

Can you telnet from this server to those domains, if so what response do you get?

Do the domain names you are sending from have spf records and if so, is the ip you are sending from included in the spf record?

Are reverse DNS records setup properly for the IP you are sending from for the domains you are sending as?

Ideally, before anyone can really give you a definitive answer on the problem you need to have a telnet or something and get a server response telling you why you cannot connect in the first place.

Solution 2:

One possible option is that these servers are flagging your email as Spam. Even if it is not really spam, many email providers block messages that are not exactly correct, or that are going out to a large number of people.

We ran into this at one point and had to contact a bunch of providers to get white listed. Once we contacted them and explained our email coupon program (opt in, fully compliant with CAN-SPAM) and gave them the details, they added us to white lists.

There are a lot of things that can get you flagged as a spammer. Here are a couple to check:

1. Number of emails going out.
2. SMTP headers on the email not matching the domain you're sending from. (Example: using a from address of @microsoft.com when the email is coming from your own domain.) This can happen by accident if you're using a domain like @yourcompanyname.com, but the SMTP server is registered under a different domain or is not registered under your domain.

Here are a few links you may find useful

http://searchwarp.com/swa209211.htm

http://www.wilsonweb.com/05/020529b.htm

http://searchdomino.techtarget.com/news/article/0,289142,sid4_gci1192720,00.html (not really Domino specific, ins spite of the url)