Might find .xyz domains are more likely to be marked as spam, and people may be unlikely to want to use one. Its short to write but long to say / not common.

I could be wrong but I think very few will be enticed by the domain alone or at all.

Personally if I was building an email platform, I wouldn't rely on a third party email API as its part of your core thing, I'd run my own email stack, would give you so much more control / flexibility and be cost effective.

I'm pretty happy with fastmail for my email and use mailpace for transaction stuff from apps, although I did run my own mail servers for about 10 years.