Error message 550

[#1]

Everyone, I am having a problem sending e mails.  I usually use a PlusNet modem at home however I am away and using WiFi through a BT Home Hub 2.0.  For the first two day everything was fine and then e mails stopped being transmitted although everything is coming in fine.  I get the following error message
The message could not be sent. The setting for your outgoing email [SMTP] server might need to be configured. To find the server settings for 'xxxxxxxxxxxxxxxxx@yahoo.co.uk', please contact your email service provider.
Subject 'badges'
Server Error: 550
Server Response: 550 <xxxxxxxxxxxxxxxxxxxxx@yahoo.co.uk> recipient rejected - relay denied
Server: 'relay.plus.net'
Windows Live Mail Error ID: 0x800CCC79
Protocol: SMTP
Port: 25
Secure(SSL): No

I have tried sending to other mail addresses of other people also staying here but get the same result. 
Can anyone please give me a clue of some help.
Many thanks.
Dave.

[#2]

Hi Dave,
Is it a Plusnet email address you're sending from and if so, what outgoing server are you trying to send your messages through? relay.plus.net?
If you are using our outbound mail servers then you will need to use authentication when you're off the Plusnet network i.e. you'll need to tell your email client to authenticate using your Plusnet username/password. See the 'Send mail from any connection (SMTP Authentication)' articles here.

[#3]

Bob,  Many thanks for your help all fixed.
Best regards
Dave.