In <list-12280221@2rosenthals.com>, on 02/20/25
    at 03:35 PM, "Lewis G Rosenthal" <ecs-isp@2rosenthals.com> said:

Hi there,

Today, att.net rejected mail from us to one of their subscribers with a
550  5.7.1 error:
   <recipient-address @ att.net>
     host ff-ip4-mx-vip1.prodigy.net [144.160.159.21]
     SMTP error from remote mail server after MAIL FROM:<sender from the
2rosenthals.com domain>:
     550 5.7.1 Connections not accepted from servers without a valid
sender domain.flph829 Fix reverse DNS for 50.73.8.217
secmgr-va.2rosenthals.com

I get the same here

; <<>> DiG 9.11.37 <<>> -x 50.73.8.217
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 42869
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;217.8.73.50.in-addr.arpa. IN PTR

;; ANSWER SECTION:
217.8.73.50.in-addr.arpa. 3600 IN PTR secmgr-va.2rosenthals.com.

;; Query time: 246 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Thu Feb 20 14:12:39 PST 2025
;; MSG SIZE  rcvd: 92

A wed search for

  "flph829"

results in a few hints that match your problem, but nothing recent.