Email Delivery Fails with the "Recipient address rejected: ERS-DUL" Error

Symptoms:

Sending emails to certain recipients fails with the following error showing up in Exim's main log: 

2020-08-23 23:48:24 1kA4P6-00C0Im-CE **mail.host.com R=dkim_lookuphost T=dkim_remote_smtp H=email@host.com [13.238.202.142] X=TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=yes: SMTP error from remote mail server after RCPT TO:<email@host.com>: 550 5.7.1 <email@host.com>: Recipient address rejected: ERS-DUL.

Description: 

This error means that the remote/receiving SMTP server, mail.host.com , is rejecting messages because they are coming from a source IP address listed in DUL. A Dial-up/Dynamic User List (DUL) is a type of DNSBL which contains the IP addresses an ISP assigns to its customer on a temporary basis, often using DHCP or similar protocols. Dynamically assigned IP addresses are contrasted with static IP addresses which do not change once they have been allocated by the service provider. Most mail servers only accept messages coming from static IP addresses to reduce spam traffic.

 
Workaround:

To remove an IP from the list, you can do any of the following:

• Contact your ISP, who will then make a request via:
  • Email: dul@mail-abuse.com
  • Webform: Global Blocked List Exceptions form
  Then they need to provide the reason why they want to remove the IP address(es).
• Configure the mail client on your own mail server to relay outbound mail through the ISP's designated mail relay. You need to discuss this option with your ISP. 
• Alternatively, you can try reaching out to users/administrators for the receiving mail server and ask to see if you can get your IP removed from the block list.