There are many, many things that can cause email issues. Here are some of the most common issues I have encountered over the years. In no particular order…
1. DNS changes. Someone may have changed the MX records, or changed the authoritative DNS servers for your domain.
2. Firewall. If the access or NAT rules for the mail server were changed, mail flow may be disrupted. Make sure you have a system in place for tracking configuration changes on your firewall(s), so that you can look at the most recent change and decide if it created a problem with mail delivery.
3. Spam Filter. After the firewall, the spam filter is probably the next hop for your mail after it passes through your firewall. Whether it is a standalone device or software installed on your mail server, check it to see if your mail is getting hung up here.
4. Services/Processes not running on mail server. If you have Microsoft Exchange, check the SMTP or Microsoft Exchange Transport services.
5. DNS resolution problems on mail server (for outbound mail). If a server can’t resolve MX records, it’s not going to be able to deliver mail. If you are running Microsoft Exchange, the primary and secondary DNS servers configured in the TCP/IP settings are likely to be two of your domain controllers. The easiest way to see if you are having a DNS issue on the mail server is to see if you can browse the Internet from it.
6. Incorrect or misspelled email aliases or domain names. This is normally a user mistake when sending a message. However, sometimes it can be due to a misconfigured contact or distribution group in Active Directory.
7. Mailbox full, either on the sender or receiver side. This will usually cause a bounceback message. If the user with the full mailbox is on your server, you will need to have some kind of monitoring set up to alert you to the problem.
8. Smart host not responding, credentials incorrect, or otherwise misconfigured. Possible ‘daily limit’ reached on number of relays.
9. Reverse DNS issues. See my previous post to make sure your reverse DNS is configured correctly.
10. Blocked by ISP. Clearwire does not allow you to host a mail server by default. Others, such as AT&T, may blackhole your WAN IP if they detect virus-like or spambot activity coming from your address. You should call your ISP to confirm if you suspect this is the problem, assuming they haven’t notified you already of the block.