Those kinds of error messages usually indicate either a wrong password or wrong authentication methods. The first thing to do would be to verify that your settings are correct. I'm not familiar with that ISP, but here's what I copy & pasted from their web site, <https://astound-prod.mindtouch.us/support/email-help/email-and-server-settings-for-all-markets> :

Outgoing SMTP Mail Servers for astound.net, CA, MD, OR and WA

   SMTP Authentication must be used with this option

   Account Type: SMTP
   Port # 25
   Security: None

   Alternate SMTP
   Security: STARTLS
   Port #: 587

For mobile access using providers with blocked ports

   STARTLS with SMTP on port 587 is currently enabled        STARTLS     587

   Domain: @wavecable.com
   Outgoing Server: mail.wavecable.com
   

=============

Email Server Information for astound.net, CA, MD, OR and WA

All email client programs need the following information:

   Account Type: POP or POP3 (single device) / IMAP (multiple devices)
   Email address
   Name of user
   Login/User name Full email address
   Email Password

Account Type: POP/POP3 Security: SSL Port #: 995

Account Type: IMAP Security: SSL Port #: 993

Account Type: POP/POP3 Security: None Port #: 110

Account Type: IMAP Security: None Port #: 143

Incoming Mail Servers for astound.net, CA, MD, OR and WA

   Domain: @wavecable.com
   Incoming Server: mail.wavecable.com

Noticed the link Lin posted does not work as expected so try this: https://astound-prod.mindtouch.us/support/email-help/email-and-server-settings-for-all-markets

For SMTP

• Make sure outgoing server name is correct - I'm not certain as it depends upon your email address, but you should get info from the link above.
• Port 587
• Connection SEcurity: STARTTLS
• Authentication Method: Normal Password
• username - usually full email address