Problem Getting the response Mail undeliverable to recipient(s) due to relay error. The socket error number is commonly 501, 553, or 554 in the email response. In Outlook or Outlook Express, you will probably also have the error number 0x800ccc79.

Solution Due to an upgrade on security to our sending mail server, some internet users will have to use SMTP authentication . The steps below will show you where to find the setting. 

Note: This is also the case with users that are connected up through another provider using a Visionary e-mail account.

Microsoft Outlook

1. Click Tools then Email Accounts (Accounts and Services on older versions of Outlook)
2. Click View and Change and Click Next (skip if it opens up to the account screen)
3. Click Change or Properties of your Email Account
4. Click More Settings
5. Click Outgoing Server
6. Check My Server Uses Authentication
7. Select Log on Using
• Type Account Name (which will be your full email address)
• Type Password (which will be your email address password)
8. Click OK
9. Click Next
10. Click Finish

This should fix the Outlook to send email. Close Outlook and re-open it to start sending mail.