Since applying the 8.103, 8.200 and 8.201 (I did them all together) updates there is now a problem with RDNS checks. I have reported this to support who has not provided a solution yet and have had to simply disable this check.
A user sending an email with this problem will get the error RDNS missing and the Mail Manager will report the same in the SMTP Log.
The problem seems to occur when a senders email comes from an IP where the RDNS for that IP does not match the sender's MX record. This is all good and well but but emails from the likes of ups.com and irs.gov are getting rejected among many others. On the first complaint it seemed like the issue had to be with the sender since other emails were getting through but after 2 days and examination of 5 updated systems it appears we have been given a new 'feature' with the only remedy for now being to disable RDNS checks.
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