Despite all of the measures that we take to maximize the likelihood of successful delivery, the problem of spam filter false-positives by over-aggressive still persists. From time to time, you may receive reports from recipients that they've not received a message that you've sent them through UltraSMTP.

In this case, the first course of action that you should take is to login to your account at https://www.ultrasmtp.com/, and check the delivery status of the message in question. If the message was not delivered successfully, view the delivery transcript for the message, and this should indicate the reason for the delivery failure. If the message was delivered successfully, the delivery transcript should show a receipt acknowledgement from the recipient's MX mail server (the mail server that handles incoming mail for the recipient).

If the transcript shows a receipt acknowledgement, but the recipient is not finding the message in their inbox, you may want to advise the recipient to check their spam or junk folder to see if their spam filter may have erroneously flagged the message as spam. If so, the recipient may want to whitelist you (the sender) as an allowed sender, to prevent the problem from happening again in the future.

If the transcript shows a receipt acknowledgement, and the recipient is not finding the message in their inbox or in their junk or spam folder, the recipient may want to report the problem to their IT department or their email provider. You can help the recipient get to the bottom of the problem by providing the recipient with a copy of the delivery transcript, so that they can attach this to their trouble report. The recipient's IT department or mail service provider should be able to track down the missing message with the help of the delivery transcript, and prevent the problem from recurring in the future.


Related:
Blacklists, Greylists, IP Reputation, and Spam Filters
Why Mail Sent Through UltraSMTP is More Likely to be Delivered Successfully
Delivery Status