Sending EMail Issues

I get the following error when trying to send emails from Outlook:

Sending' reported error (0x800CCC6F) : 'Your outgoing (SMTP) email server has reported an internal error.
If you continue to receive this message, contact your server administrator or Internet service provider (ISP).
The server responded: 554 cxr-obgw-5002a.stratus.cloudmark.com cmsmtp 152.37.154.144 blocked.
Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information.
AUP#CXDNS'

I can send email's fine from Cox Webmail.

Any suggestions?

Outlook version: 

Microsoft® Outlook® for Microsoft 365 MSO (16.0.14228.20200) 64-bit