AUP#Out-1500 send failure, Spectrum ISP

Recently used Starlink connection on a trip, I could only send on first registered Spectrum (@twc.com) email address, not on the secondary twc.com email address. When got home and connected to my Spectrum ISP, could send again on both addresses for few weeks.
Last week reverted to AUP#Out-1500 msg when sending on second email. Spectrum 'tech' no help. Using TB 3.1 since 2012 with no issues. Not a mobile user so have not wanted, nor needed, to upgrade.
Like 3.1 reliability, simplicity, speed; so would like to retain if this can be solved. Only the SMTP password for primary (first registered) address is present in master list of passwords. 3.1 has no way to manually enter SMTP password.

Would TB 52 solve this issue, ie allow client-side entry of SMTP password? If not, would any recent TB solve this? Or am I looking for a new ISP?

TIA,
Strong open source supporter since first accessing the internet via Lynx on the Univ. of Buffalo Free-Net.