On the Recipients To Name, if the name have some characters, like ´ or ç the user receive an error like Undelivered Mail Returned to Sender. Something like this. This issue only happens to users that have strange characters.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

host outbound.mailspamprotection.com [108.163.220.50]
SMTP error from remote mail server after end of data:
550 To contains invalid characters.

Hello Manuel,
I've tested this and cannot really see an issue. Are you sure the recipient's server accepts usage of UTF8 symbols in the Name?

The user fiils the form and when submit it sends an automatic email. The field name on the form is automatic populated according to the login user. All data bases are utf8 unicode. I have place on the send mail the field name of the user, and this only happen when the name of user has some strange characters..
I'll try to retest everything again.
Do i have to apply the unicode to that field (name) on the html5 validator ?

No, this is not needed. I am asking about the recipient's server. Did you test on another server as well as i tested it with several SMTP servers, and everything went through just fine.

Hi Teodor. My hosting provider is siteground. I have my website hosted there. I can't test it on another server. Is there something that i could ask siteground in order to solve this?

What i mean is try sending the same mail to different mailboxes gmail, yahoo etc. do they return it also?

Hi. Most of users, use the emails provided by their companies and that is what's happening ...

Well, then there must be some issue with their email servers ... Please do check this with your own email and see what happens.

With my email i don't have problems ... the problem is that there are too many users each with their own company and most of them have this issue... the way i solve this is replace the name field with other field that don't have strange characters.
But, i really wish to solve this the right way

Well, this is not related to our product... if their servers do not accept UTF8 chars we cannot change this...
What you can do is to use the validator to validate the input to not allow such chars, so when they use Unicode chars it will display an error, and they should change this to non-unicode ones.