Trouble sending invoices as emails

Hi, I am recieving this error when trying to send invoices by email. This means I must print each invoice off and scan them into the computer so that I can get it to the recipient. The error reads: An unknown error occured connecting to the SMTP server. The server may be unavailable or refusing SMTP connections. If your webmail address and settings are correct, you can re-enter your password and try again. 

I am using an Outlook email address (ends in hotmail.com) is this the issue?

Any guidance is greatly appreciated!

Cheers