InfinityFree doesn’t give you email access right out of the box sadly… To use emails with your domain, you need to use a third party software. I am using yandex for my domain.
Check this:
YANDEX XHANGED ITS UI AFTER THIS POST WAS MADE SO TO SIGN UP, YOU NEED TO GO TO PRICES AFTER GOING TO CONNECT.YANDEX.COM, AND THEN SELECTING THE FREE OR OTHER PLAN(S). Then it is similar to the instructions