E-Mail headers with `@` trigger some spam filters

[techknowlogick]

Originally posted in #28981 (comment)

cc: @wxiaoguang @gwymor

[techknowlogick]

pinging @pat-s too.

below quote is from @gwymor in linked issue

What displays that message? Are you aware of email providers/stacks that will take that as a sign of spam/phishing and flag the message? I would hope nothing would make that mistake as there's no user-part before the @.
If email providers do, we should probably drop the @, and display the sender as Display Name (username).
When a user has no display name the sender currently falls back to @username. If we change that to just username, I'm afraid that doesn't make it clear enough that this is a username that cannot be spoofed (aside from inspecting the X-Gitea/GitHub-Sender headers, which I think is too much to require a user to do in a spoofing attempt) -- you can't easily tell the difference between a post-1.22 Gitea sending a username, and a pre-1.22 Gitea sending a displayname that looks like a username. We could remove the fallback code, and always show Display Name (username), which will fall back to username (username). That would be more consistent and easy to verify at a glance, but perhaps noisy.

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