Mail Servers FAQs - If you are using your own mail server program to send emails the most common problems for undeliverable mails are identified here, please go through them carefully and it will solve most of your problems.

Most of the time, undeliverable emails to recipient's mail servers are due to your mail server identifying itself as an internal mail server with an IP address 127.0.0.1 and can be solved by using an actual IP address?

Actual IP Address?
For example; using invalid, non-existing, email address in the REPLY TO: or FROM: field or web-based free email address with no MX Records or the email address at the FROM: field that is different from the login IP address. See: Email Filtering

Every ISP has their own standards and ways to filter spam, therefore, by clearly identifying yourself at the FROM: field for most web-based email recipients like: Hotmail, Yahoo, etc., may not work with AOL or other ISPs or the other way round.

You may need to segregate your email address lists, as well as to keep in mind that ISPs also limits accessibility to their mail server by time and quantity of mails (whether you are using your own mail server or ISP mail server).

Therefore, you may need to experiment by using a combination of different strategies for different popular web based email recipients.

Notes:
ISPs DNS server IP Address has nothing to do with the process of email message delivery. It merely serves to turn the IP address--a series of four 8 bit numbers, ie: 151.196.75.10 into a domain name www.me_my_domain.com