Email does not comply with addr-spec of RFC 2822

Can you debug to see which validator is acting on the address?

If the email contains <> it should also have a name, e.g Bob<bob@example.com>

Thanks for the quick response.
The validator is Egulias\EmailValidator\EmailValidator.
I see that it indeed tries to send a <mailaddress> once with and once without a name.
For verification, I sent a mail through another website and there I got no error message. Difference: the use of mimemail. I will report the error to that module.

That is the library, yes. But I was asking which of MessageIDValidation or RFCValidation is acting on that message. I sent a link to a line of code as a reference.

It is the MessageIDValidation

For your information, as a test, I modified the code with an extra line: if (empty($name)) $name = 'info@kunstbehoudgroningen.nl'; before $this->name = trim...
The error message remains the same.

I had the same issue on last version (10x) install, to solve it, apply "Email address" on the Basic site settings page (/admin/config/system/site-information).
But, why hasn't the email been applied after the Drupal install - this is a need to investigate I think.

The < and > chars are the problem here.

A workaround is adding this line:
$this->address = trim(str_replace(["<", ">"], '', $address));

Still not sure where these are added though, the users_field_data db tables does not contain them.