I'm not sure. I don't like @abc.xyz.com at all. @xyz.com/abc has the nice benefit that it is like https://xyz.com/abc https://xyz.com/abc with only the https:/

I'm not sure. I don't like @abc.xyz.com at all. @xyz.com/abc has the nice benefit that it is like https://xyz.com/abc https://xyz.com/abc with only the https:// https:// part is changed. But "Written by @joe@someinstance.com" reads much nicer than "Written by @someinstance.com/joe".