It's likely that that mail server is 'internal' to their network, so try port 25, with no SSL and no 'Use default credentials', just user id and password.
Hi, so sorry for the delayed reply - Everleap seems to have resolved my issue on their end, something about an MX record adjustment. After the MX change, and before I ran another email test, I also re-input the password for the admin email account (just to be sure) and saved.
All tests worked after that. :-)
I really appreciate everyone's input. nopCommerce has such a wonderful support forum, and I am truly grateful for your time!