In many cases our mail servers reject connections that don't follow the RFCs for mail delivery.
The most likely cause is that your smtp SENDING mail server or mail sending system is not correctly configured. In years past the general internet best practices where along the lines of the famous "be conservative in what you send" and "be liberal in what you accept" philosophy.
We (along with most other mail providers) longer think it's applicable to an inbound SMTP server however. The more your SMTP sender looks like a spam source the more likely other (not just Richweb's) servers will reject, defer or quarantine your traffic.
Missing rDNS (reverse dns or ptr records) for your mail server sending IP address, incorrect HELOing (i.e. your mail server saying HELO as something invalid like server1.local) or your system not sending valid SMTP commands
are all reasons that your traffic may be rejected.
It is in everyone's interest to make sure their outbound SMTP traffic is as compliant with de jure and de facto RFCs, and BCPs as they possibly can.
Sometimes we hear the "but this works with hotmail or provider X or ALL my other customers" response. Other providers have different rules for incoming mail; some are more lax than others.
Some things to look for on your system:
1. AOL is probably a better example to follow than some other providers in terms of increasing strictness. AOL will reject email that is not rfc compliant - for example missing brackets:
250 rly-dc04.mx.aol.com OK
mail from: test@test.net
501 SYNTAX ERROR IN PARAMETERS OR ARGUMENTS
mail from:
250 OK
2. Another common mistake is the HELO misconfiguration.
If your mail server dns name is mail.domain.com, but your mail server says HELO (greets) other mail servers as server1.local (which is fine as an Active Directory DNS name, but NOT usable on the global internet) your mail will have problems. Make sure your mail server has a HELO that matches its fully qualified (hostname + domainname) domain name.
Correct example:
telnet mail.richweb.com 25
Trying 63.90.9.3...
Connected to ford.
Escape character is '^]'.
220 mail.richweb.com ESMTP Postfix
3. When you check the reverse dns for Richweb's mail server you will see that the IP resolves to the hostname:
dig +short -x 63.90.9.3
mail.richweb.com.
This is a MUST for proper mail sending. If you do not have correct reverse dns in place many large providers will block or delay your email.
4. SPF records (if used) must be correct. All of the servers that can send mail on behalf of your domain must be listed.
Here is a correct example for Richweb's domain:
dig +short txt richweb.com.
"v=spf1 ip4:63.90.9.3 a:vsmx3.richweb.com a:mail.richweb.com mx:richweb.com ip4:63.90.9.6 -all"
That says these servers:
mail.richweb.com (main mailbox server)
63.90.9.3 (ip of main mailbox server)
63.90.9.6 (ip of customer relay server)
vsmx3.richweb.com (mail filter servers)
are the only servers on the internet that can send mail that claims to be from @richweb.com.
Summary:
If you need help with your HELO, ptr, or spf records contact Richweb for assistance.
All mail systems are increasingly restricting the flow of email from improperly configured systems. So while your mail flow may not break to other systems today; without change it will probably break tomorrow if you are not standards-compliant.