I have a Google Mail account set up to receive and send messages via the Outlook.com servers. Receiving works fine, but sending has recently started bouncing back with this error:

   Delivery to the following recipient failed permanently:
   
        (recipient email address)

   Technical details of permanent failure:
   Google tried to deliver your message, but it was rejected by the relay smtp.live.com by smtp.live.com. [65.55.162.200].
   
   The error that the other server returned was:
   535 5.2.1 Login to your account via a web browser to verify your account. (SMTP AUTH failed with the remote server)

Sending directly from the Outlook.com webmail works fine, as does sending via an SMTP client on my computer (I used Office Outlook without the Hotmail connector to test it). This problem appears to be confined only to sending via Google Mail.

I've checked all the settings (server = smtp.live.com, port = 587, secured via TLS) and reentered my email address and password, but to no avail.

I'm not really sure what the error message means. Is it saying that I simply have to log in to the Outlook.com webmail to verify the account? Because I've done that and it didn't change anything. Or is there an additional verification page somewhere that has to be used? If so, I can't locate it.

Can anyone please advise how this may be fixed?