Following up The Builder's advice:
Go to Tools > Accounts.
Click on the name of the relevant account, to highlight it.
Click Properties.
Look for a box marked 'enable SMTP authentication' (or something similar), and see if it's got a tick in it. [If it hasn't, try putting one in it].
If that doesn't work, try deleting the account and then creating it again. (Instead of clicking Properties, click Delete and confirm your choice. Then Go to Tools > Accounts again and use the option to create a new account).