Fediverse addresses have two parts, the username and the server name. However, by default Mastodon and some other Fedi server types hide the server part of the address within posts to avoid looking too messy. This means that it’s a good idea to choose something distinctive for a username, so that when people see your account mentioned in a post it is clear that you are being talked about.
For example, if there was an account with the address @MarvellousWidgets@example.com, only @MarvellousWidgets would be visible on posts, though clicking on it would lead to a profile page where the full address appears. This is a good distinctive username because people can refer to it in discussions easily, and a post might say “Have you seen the latest release from @MarvellousWidgets? It’s very useful.”
On the other hand, if there was an account that had a much more generic username such as @email@example.com, all that people would see of its address in posts would be @software which would be very unclear and unmemorable. A discussion would look like this: “Have you seen the latest release from @software? It’s very useful.”. People would have to click through the username each time to know who you are talking about.
Those users on their own servers may think their custom domain gives them enough distinctiveness. However, as people don’t see the domain part of Fedi addresses most of the time, even people on their own domain need to think about distinctive usernames.
Don’t use dots / full stops / periods in usernames
Some Fediverse platforms allow you to use . in usernames while others don’t.
If you use dots in your username, it may make it impossible for you to be followed from some types of servers, including Mastodon servers.
The safest option in this situation is to just completely avoid using dots in usernames.