We recently learned of a problem affecting more and more email users - not just bulk email users, but all users! Once again, the problem is attributable to... er, improvements in spam prevention and filtering. Here's what's happening:
You send an important email to someone, and even after a reasonable amount of time passes, they tell you that they never received it. The first thing that you are likely to tell them is..."please check your spam or junk-mail folder - it's probably in there." They check the spam folder, but it's not there either. Where did it go? Well, there is the possiblity that their server is ignoring or deleting suspected spam before it reaches their email account. For this, you can easily have them add your company to the mail system's whitelist to solve the issue.There is another possibility, however, that many people don't know about, called sender/receiver verification (hereafter we'll just call it sender verification or verification).
Sender verification is a simple mail server setting that has been implemented to help avoid the waste that spam creates. If you send an email to a customer whose email server has sender verification turned on, their server will send a request back to your server to verify the sending account actually exists, before distributing the email to the appropriate account. If there is no response from your server to their server's verification request, or the email account is indicated as non-existent by your server, then the email is dropped, lost, gone!
Well...this is fine if the email account doesn't exist, because you don't really have a good reason to send email with a non-existent reply-to address, do you? If your server IGNORES the verification request, however, then you get the same result - dropped email. Therein lies the problem. Legitimate email can be dropped simply because a sender's or recipient's server doesn't respond appropriately to verification requests. Whitelisting the domain won't help in this case. Instead, you'll need to ensure that your email server responds to sender verifications in order for your recipient to get their email.
Wait - there's another scenario! If your server responds to verification requests, then the problem in this scenario could be that YOUR server requires verification, but the recipient's email server doesn't respond to verification requests appropriately. In this case, you'll either have to turn verification off in YOUR email server, or get the recipient to turn on verification responses in their email server.
If you attempt to send email to someone, and get a bounce-back message that includes a statement similar to the following, then the problem is very likely with YOUR server requiring verification:451 Could not complete sender verify callout.
Finally, some verification algorithms check the domain of the recipient to ensure that reverse DNS is set up (the ability to get a domain name from an IP address). If it's not set up, then verification could fail with an error similar to the one shown above. Make sure that reverse DNS is set up for your server.
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