It is not that easy. Several requirements need to match to make this possible, the software needs to support it, the mail server needs to support it.
Also, this is a new feature, not a bug.

Postfix supports this since v 3.0, however PHPMailer currently does not support this at all (see https://github.com/PHPMailer/PHPMailer/issues/49 )

Wow, I wasn't aware that these tools do not allow this. According to RFC 5321 and 5322 the accent is an allowed character, see also https://en.wikipedia.org/wiki/Email_address#Local-part.

So what do you suggest how to deal with this? We have seen such email addresses at larger user lists a few times.

@Mazi Where do I find the passus which ensures full support for accents without restrictions?
I don't see RFC 5321 and 5322 as relevant.

Take a look at https://en.wikipedia.org/wiki/International_email .

The wikipage states
"In addition to the above ASCII characters, international characters above U+007F, encoded as UTF-8, are permitted by RFC 6531 when the EHLO specifies SMTPUTF8, though even mail systems that support SMTPUTF8 and 8BITMIME may restrict which characters to use when assigning local-parts"
which means that it is up to the mailserver to signal what they support.

If someone uses accents in their main email address they won't be happy in the long run.

The warning is important and correct.

@jelo, thanks a lot for sharing your thoughts. I agree that this can be considered a correct validation. I think the main question then is: Why do some companies create such email addresses? Just a lack of background knowledge?

@c_schmitz, I am happy to close this one if you think no changes are required.

I check on iredmail on LDAP seems : didn't accept it for account but seems partially OK : https://forum.iredmail.org/topic15252-postfix-smtputf8-support.html?login=1