There's a reason why most of those valid email addresses are not allowed by most email providers. As an example, in the 1997-early 2000's, underscores were very popular amongst the internet users and most of them had an underscore in their E-mail addresses. Whilest applying for a job, they would fill out a form in the employer's website/some form application software where the data is then later tabulated.

Once tabulated, the entire email address would be underlined by many popular softwares back then, since it's (was) essentially considered as a link. Then, while the recruiters were trying to copy and paste these prospective e-mail ID's in their respective email clients, the underscore would be missed out and would have just a space instead (which the recruiters have no idea as to why) and thus the recipients would miss out from receiving these emails. Hence, many email providers wouldn't allow one to register an underscore (or any complex character) after receiving many such reports, just to avoid these hassles.