Symptom: Inbound email to example.com domain was not being delivered, outbound email from example.com was OK.


Cause: SMTP Recipient checks enabled but not supported by destination SMTP delivery server


Specific Details:


[xxx.xxx.xxx.xxx] F=<sender@example.com> rejected RCPT <recipient@example2.com>: The email address does not exist

H=(sendingserver.example.com) [xxx.xxx.xxx.xxx] X=TLS1.0:DHE_RSA_AES_128_CBC_SHA1:128 F=<sender@example.com> rejected RCPT <recipient@example2.com>: The email address does not exist


By default, MailShark is setup to check recipient addresses on SMTP delivery servers ie. under "Domains -> example2.com" is the option "SMTP recipient checks" which is enabled by default.


This feature checks that the email address exists on the SMTP delivery server by sending a SMTP greeting tuplet (HELLO, MAIL FROM and RCPT TO). If the SMTP delivery server responds with "User unknown" then MailShark will respond with "The email address does not exist", as shown in bold in the string above.


If the SMTP delivery server responds with "Recipient OK" then MailShark will send the email through to the SMTP delivery server.


Why have SMTP Recipient checks?


SMTP Recipient checking is an Anti-Spam feature which reduces load on the SMTP delivery server from handling thousands of emails and producing unnecessary Non-delivery Reports (NDR's). SMTP Recipient checking also eliminates "Backscatter" by not producing NDR's to non-existent or faked Sender addresses.


SMTP Recipient checking is a rock solid feature but can be easily disabled by de-selecting the option in the MailShark web interface.


At MailShark, we have never (ever) seen this feature cause a problem and should only be disabled if the SMTP delivery server does not support recipient checks.


All modern SMTP servers support recipient checks, however may sometimes need to be enabled as the default setting may be to accept all recipients (this is Microsoft Exchange's default behaviour).