To see if your mail server did not fall in a blacklist when sending an email you can check the following website:
http://www.mxtoolbox.com/blacklists.aspx
Other sites chef year the same job:
http://www.dnsbl.info/dnsbl-database-check.php
http://www.anti-abuse.org/multi-rbl-check/
http://multirbl.valli.org/lookup/
To verify that our IP is not generating strange traffics you can use this Cisco tool:
http://www.senderbase.org/
To ensure that our domain has the MX record and other DNS parameters correct, you can use:
http://www.intodns.com/
http://hostadvice.com/tools/whois/
Once you excluded that our server is entered in a blacklist, which does not generate abnormal traffic and the DNS registration is correct you should verify the functionality trying to send email externally:
You can use the following email servers external to check everything:
http://www.mxtoolbox.com/diagnostic.aspx
http://centralops.net/co/EmailDossier.aspx
http://multirbl.valli.org/email-test/
You can utilize this tool:
http://www.mxtoolbox.com/
http://centralops.net/co/NsLookup.aspx
Another control that can be done is that Internet servers are operating properly and that it is not an isolated is that prevents communication between mail servers.
To do this you can use one of the following links:
http://www.internetpulse.com/
http://www.internettrafficreport.com/
Here you have a website to quickly send emails:
http://davidp.ordinarylight.com/qm/
[Italian Article link: http://www.alessandromazzanti.com/2012/03/server-verificare-il-funzionamento-di.html ]