Relaying not allowed?

Getting an error whilst emailing like this?

Relaying not allowed. Please use SMTP Authentication

If you are trying to send an email to someone @ then double check that you are not trying to send to as both domains exist but only the o2 domain accepts emails. Ensure you are using o2 as in lower case letter "o" and the numeral "2" rather than numerals "0" and "2"

If you get this error and it's not to o2, then double check that the domain/email address actual exists. One good site to check for this is