Possible Reasons - End users are not receiving emails sent from Freshdesk due to one of our NAT IPs in the blacklist.
Resolution Path: NAT IPs will be used when a custom mail server is used for outgoing emails from FD. Here is the list of NAT IPs.
-- Get the affected email address, ticket ID, and email sent time stamp from the customer.
-- Access Freshops > Check POD > Open sumologic > Paste email address, right time frame > & search for email delivery status.
-- If "blocked, blacklisted, black ip, or block listed, deferred” is found in "rsn" the email is blacklisted. Copy the “ip” and paste it into MXtoolbox to see who blacklisted the IP. It could be by Spamhaus, Sophos, FortiGuard AntiSpam, or others. 
-- Open fp-email-help and paste this IP to check if there are any recent open threads for this issue to understand the status. If you don’t find any reports send a new message in this channel with all details (IP address:, blocked by:, MXtoolbox link, etc) to resolve the issue.
-- If you are unable to locate any hits in Sumologic, use the “freshemail” log set in the Haystack application to get the error response.