Please be mindful that you responded to a post citing the example of

  ""()<>[]:,;@\\\"!#$%&'*+-/=?^_`{}| ~ ? ^_`{}|~.a"@example.org"

That address is not "common" even if it is technically valid. If you want to talk about your own email address, you should provide a sample address that is similar so we know what you're talking about.

[edit: I see your address in your profile. It is nothing like the example given.]

If it's the one that you have on some of your social media sites, nice. I can suspect why they reject it but is it primarily because of the short length?

At a guess the problem is probably a code snippet that checks for minimum length. You could have an even shorter one for a single letter domain in a two letter tld, for instance [email protected]