won’t change Mail Userid to email address

Using latest Postie (v1.9.56). On mailserver settings page, will not let me enter an email address in the “Mail UserId” control. It shows the admin username instead of an email address. I enter an email address and “save changes” but it simply goes back to admin userid. Changing other settings at the same time (like port) all are changed as requested. Trying different email address does not work either.
I was able to get it to work in a test environment a few weeks ago with same settings.
Am now trying to set up and test on a new production site, starting with empty Hello World wordpress.
server: Hostgator
Wordpress: 5.8
Theme: Twenty Twenty-One
Other plugins: Duplicator, Ultimate Blocks, User Role Editor