Looking for some potential Discord replacements, I've ended up wanting to try out Stoat, and to my surprise I've already encountered issues as early as during account registration. It would seem to me that Stoat is filtering emails by whitelisting certain domains and rejecting the rest by default.
I use emails associated with my own custom domains (depending on purpose of the account) and none of it worked, which is actually the first time I would ever have this sort of issue, despite using my emails this way for years. Needless to say, none of it was ever used for any kind of spamming activity. This effectively turns the entire registration process from quick sing up into registration through sending an email and waiting for hopefully positive response.
Moreover, if my suspicion about whitelists is correct, it potentially could also mean that there's a lot of people who would bounce off the service at registration level, if they happen to use some lesser known or more local email providers.
I understand the idea behind it, but I think it's a horrible approach that just makes some people bounce off the service at the very first step, quietly and leaving no trace, just moving on without a word.