“Your message couldn’t be delivered because you weren’t recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it’s no longer allowed to send email. Contact your email admin for assistance.”
This is an error you get when your anti-spam “outbound” policy restricts the user from sending email. These settings are set in the “Recipient Limits” tab of the outbound policy as show:
In the above, the user is set to no more than 20 external emails a day. This will trigger the above error message on the 21st email and later!
The default settings, which would fix this error, are shown below – but you can set any settings you need, but obviously if the values are too low you will get the above error message.
The default policy for recipient limits is 500 external per hour, and 1000 internal and daily limit – with the action being to restrict the user. This means the user, if they hit the limits (which are now higher than the low example above) will be able to send emails after the hour.