This error indicates that there is a problem with the mail account being used, and will need to be investigated by whoever supports your mail server.
To find out the details of which server / mail account is being used:
On the System tab, click Options, System Control Options.
On the System Format tab, click System Email defaults.
Click the SMTP Users, yellow icon and make a note of the credentials currently being used.
Check that the credentials for the user are correct. If you are sending emails out of Dimensions via a shared mailbox, please also check the credentials for this account.
Pass these details onto whoever manages your SMTP server for further investigation into the issue. Ensure that the port specified in the configuration is open in any Firewall.