Emailing an "O2" address can result in a "Relaying not allowed" error message

December 2008

It seems that UK mobile phone company "O2" doesn't just own the domain, but also the domain (with a zero instead of the letter o).

However, if you email the latter domain, you may receive an unhelpful error like this:
550 RCPT TO: Relaying not allowed -
    please use SMTP AUTH
If so, you may want to try simply replacing the digit '0' with the letter 'o'.

For example, if emailing resulted in an error, you could try instead.

I created this page because I didn't find any other info on the Web about this minor issue. Hopefully it will help other people who encounter the same problem as me.

Full delivery status message

Here is an example of what the full delivery status message (containing the above error) might look like:
Reporting-MTA: dns;
X-Postfix-Queue-ID: 1A0A11AAAA1
X-Postfix-Sender: rfc822;
Arrival-Date: Mon, 29 Dec 2008 05:17:16 -0500 (EST)

Final-Recipient: rfc822;
Original-Recipient: rfc822;
Action: failed
Status: 5.0.0
Remote-MTA: dns;
Diagnostic-Code: smtp; 550 RCPT TO: Relaying not allowed -
    please use SMTP AUTH

Side Note

I am not an "O2" customer, and am unlikely to become one in the future, due to its stupid name.

Peter Ford