Usually when an SMTP server attempts to send an email to your domain it looks up the MX record for your domains mail server to find the IP address of your public mail server. If this SMTP's DNS server does not have your domains MX record cached(i.e. it has looked it up recently) then it will attempt to contact your ISP's DNS server to get the MX (mail exchanger) IP address. If your ISP's DNS is down then when the remote SMTP server tries to send the email it will not be able to find your mail servers IP address and the email will fail to be delivered. Most if not all ISP's tend to have at least two DNS serverss so if one fails the other is still available. If you have your ISP's DNS server addresses you can check several things:
1. Are their DNS servers available, if you have the IP address of their DNS servers (they should have this on their support site, or they will let you know if you call them) you can try pinging these servers. (assuming you have internet access)
2. Is the MX record available by using nslookup from the command line.
Open a command line
type nslookup
at ">" prompt type "set query=mx" to set the query type to mail exchanger.
then "Server=dns.abc.com" replace dns.abc.com with your ISP's DNS server so that the query will query your ISP's DNS server.
then type your domain name at the ">" prompt
It should return something like:
> axsone.com
Server: dns.abc.com
Address: 194.119.131.65
Non-authoritative answer:
abc.co.uk MX preference = 50, m
abc2.co.uk MX preference = 50, m
abc3.co.uk MX preference = 10, m
abc.co.uk nameserver = ns1.demon.co.uk
abc.co.uk nameserver = ns2.demon.net
abc.co.uk nameserver = ns0.demon.co.uk
mail-1.mail.isp.net internet address = 123.123.123
mail-1.mail.isp.net internet address = 123.123.124
mail-2.mail.isp.net internet address = 123.123.125
mail-2.mail.isp.net internet address = 123.123.126
ns0.isp.co.uk internet address = 121.121.1.60
ns1.isp.co.uk internet address = 121.121.1.61
ns2.isp.net internet address = 209.124.125.110
If all emails are bouncing to your domain either:
1. Other mail servers cannot access your mail servers IP address. Test this by pinging your mail servers public IP externally.
2. Other mail servers cannot resolve your MX mail exchanger record on your ISP DNS servers (or yours if you host your own DNS).
3. The E2K server is bouncing messages because of misconfiguation or a fault.
You can diagnose the issue more if you get a copy of the bounce messages external users are getting. It is worth having a web enabled external email account (On an external ISP) that you can either get external users to send the bounce messages to or you can send a message from to your internal account and see what the bounce error is for yourself.