Windows 7: Can't send emails to a Gmail account suddenly

Hi all
I have been able to send email to my brothers Gmail account no problem until just a few days ago, & I've no idea now what has happened to cause the glitch.

Basically now what happens is that when I send the mail, it's bounced immediately back with the msg:

......... The following recipient(s) cannot be reached:

Steve () on 21/08/2012 23:25
None of your e-mail accounts could send to this recipient.




Now, what's really bugging me is that the full email address is put into the email yet when sent it strips the address out leaving just his name. I've tried every variation I could think of & have manually typed it in, deleted & restored the contact details, all to no avail. It appears to only be this one address as I can send to my own Gmail addy ok.


He's receiving mail fine from everywhere else & I can send to him if I use GMail directly but would still like to do it through Outlook if I could.

I'm using an IMAP connection for the Gmail, but as the error msg says none of my email accounts can get a msg sent (I've 8 accounts by 5 ISPs).


Any suggestions?
Thanks