I believe the Common Application accepts any email address, given that schools use .com, .edu, .state etc. Is the problem with the email to create the recommender’s account? If so, IIRC there’s a button you can click on the recommenders tab of any college where you’ve assigned the recommender to re-send the request, and ask the recommender to check their spam email.
I would not recommend submitting via snail mail, UNLESS the college only accepts supplemental recommendations that way (like Brown.) There are too many addresses to keep track of, and most schools want everything electronically submitted via the Common App due to it auto-syncing with the schools that use Slate (and especially this year given that many admissions offices are closed. If it’s truly necessary, I recommend checking with the admissions website/office first. It might be possible to have it submitted via email to the main admissions email, like at Brown where it says so in the application FAQs.
Regarding mailing addresses, some schools (like CWRU,) have separate application material processing facilities listed on their website. Most don’t, so you can just send the letter (if email isn’t fine, but it should b/c of COVID as long as it’s sent directly by the recommender and NOT from you,) to the mailing address or contact address listed on the website. If you have any questions, again, contact the admissions office to be sure: that’s why they’re there!
Hope that helps! Good luck with admissions!