Unable to Send Email

Have not been unable to send email for a month.  Just stopped working one Friday night.  Have been able to send email fine before and since.

Using Outlook to send/receive email.  Server is mobile.charter.net.

Have contacted Spectrum muliple times.  Have confirmed server settings.

I do not have this problem on other home networks.

Have performed a factory reset on the router.

When bypassing the router and connecting the computer directly to the cable modem, the issue goes away.

Cable Modem is a Motorola TM1602A MTA. 

I have not performed any updates on the router.  As part of the trouble shooting, I performed a factory reset.

All other functions are working fine on the router and internet browsing is fine.

Error message from Outlook:

'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 impout003.msg.chrl.nc.charter.net cmsmtp 75.181.209.43 blocked. Please see https://www.spectrum.net/support/internet/understanding-email-error-codes for more information. AUP#Out-1130'