Sometimes, valid emails are rejected, preventing people to register.
These addresses are actually rejected too by online checking services like http://tools.email-checker.com/
This is certainly due to the mail server configuration that does not like to be checked and sends back weird insults that makes this module mad.
Nethertheless, this should not block the registation process as it is said in the module page.

The error I get is yy@xx.com is invalid, please contact us for clarification.
This seems to comes from the last check

if (!ereg ("^250", $to)) {
    watchdog('email_verify', "Rejected email address: $mail. Reason: $to");
    return t('%mail is invalid, please contact us for clarification.', array('%mail' => "$mail"));
  }

What is this test about, why is it blocking without further explanation?

Thanks

Comments

The error is
Rejected email address: yy@xx.com. Reason: 550 5.5.1 Protocol error

I think that the module might send several commands while the server might be configured using pipelining tests.

So possibly 2 problems
1) email_veryfy ight not be smart enough to recognize this setting and serialize the commands
2) this shall not block the email check process, just warning.