Email export is blacklisted by other mail servers or firewalls

Question

I have an Exchange 2019 environment (upgraded from 2016). There are three external IP addresses involved in email communication. X.26 is used for incoming emails to the Barracuda anti-spam gateway, X.27 is for incoming access such as POP, IMAP, HTTPS, and OWS, and X.28 is the outbound email address. I haven't set up reverse DNS (PTR) records for X.28, and the post (presumably SMTP) is configured with the FQDN associated with the sending connection. In the previous Exchange 2016 setup, the outbound email used X.26, but after migrating to Exchange 2019, it now uses X.28. However, many of my users have reported that some external emails are not going out and are stuck in the queue (I've checked, and it's likely being rejected by the recipient's server). I'd like to know if my topology is configured correctly or if any changes are needed."

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