Classic has the same bugs it always did, New is OWA in a browser app window and is missing features a lot of people care about.
Either one can have a wide selection of connection and authentication issues that are more or less unforgiveable but nobody seems to care because MS is really the only one who has the entire kitchen sink in their product reasonably well integrated.
It's also the last thing I'd have sent on this mission. I guess with modern communications it's nice to have an email client on your spacecraft, but with the lag you're not using Teams and you're not going to be attending any meetings. Do you really need all the extra crap?
I'd rather have them running older, more robust hardware with more efficient and more stable code on them than anything Microsoft provides.