Issue

Users with IMAP mailboxes hosted on one.com are getting 421 bounces when sending emails to BT Internet customers:

This is the mail system at host mailrelay5.public.one.com.

####################################################################
# THIS IS A WARNING ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE. #
####################################################################

I’m sorry to have to inform you that your message has not yet
been delivered to one or more recipients. It’s attached below.

<email@btinternet.com> delayed: btinternet.com->MX:
421 Too many messages (1.5.7.1) from 91.198.169.199
Reporting-MTA: dns; mailrelay5.public.one.com
Received-from-MTA: smtp; [192.168.3.86] (81.133.42.155)
Original-Envelope-Id: 6a014cd9-a323-11e5-be90-b82a72d03b9b
Arrival-Date: Tue, 15 Dec 2015 12:16:26 +0000

Final-Recipient: rfc822; email@btinternet.com
Action: delayed
Status: 5.4.7 (Message could not be delivered in the allotted time frame)
Remote-MTA: dns; btinternet.com->MX
Diagnostic-Code: smtp; 421 Too many messages (1.5.7.1) from 91.198.169.199

Emails do get delivered eventually, but after few hours delay.

Resolution

To resolve the issue I had to add following DNS SPF entry (TXT type record) for the affected (sender’s) domain:

v=spf1 include:_custspf.one.com -all

If your email is not provided by one.com, the SPF entry would be different. For example, Office 365 customers may need to add:

v=spf1 include:spf.protection.outlook.com -all

 

December 2015