I run a mail server on a DO droplet (postfix MTA). A few days ago, Comcast stopped accepting mail from my mail server. It appears that my server is not on their block list (I've tried to remove it from their block list, but they tell me it is not on it). Instead, I get the following error when sending to any Comcast address:

Action: failed
Status: 5.1.0
Remote-MTA: dns; mx1.comcast.net
Diagnostic-Code: smtp; 550 5.1.0 Connection is not being accepted at this time.

This seems to fail after Comcast receives the FROM (my actual domain and IP address removed):

[root@mydomain postfix]# telnet mx1.comcast.net 25
Trying 96.114.157.80...
Connected to mx1.comcast.net.
Escape character is '^]'.
220 resimta-po-26v.sys.comcast.net resimta-po-26v.sys.comcast.net ESMTP server ready
EHLO mydomain.com
250-resimta-po-26v.sys.comcast.net hello [MYIPADDRESS], pleased to meet you
250-HELP
250-SIZE 36700160
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-STARTTLS
250 OK
MAIL FROM: chad@mydomain.com
550 5.1.0 Connection is not being accepted at this time.
Connection closed by foreign host.

A web search shows that I am not the only one having this problem:

https://www.reddit.com/r/digital_ocean/comments/9rypsh/anyone_else_having_issues_sending_email_to/

All of us using Digital Ocean for our mail servers. The OP there even tried creating 2 extra droplets and found the same problem at all of them.

It appears that Comcast is refusing connections if the domain of the FROM address resolves to a Digital Ocean IP address.

Are others seeing this? Does anyone have a good work around other than taking my mail server away from Digital Ocean? Does anyone know of a way to contact Comcast to complain? As I said, I've tried submitting their block list form, and they just tell me my IP address is not on the block list.