[exim] From field with comma rewritten

Hello,

When a message with a 'From:' header line contains a comma, like :
    From: my, test <my@???>
Sendmail rewrites it, prepending @domain_ name before the comma :
    From: my@domain_ name, test <my@???>

If the line is encoded beforehand by the SMTP client (quoted-printable,
base64, ...), this rule does not apply. I noticed the same behaviour with Sendmail.

Is there an explanation to this, and a way to deactivate it ? It's not
about the MAIL FROM command, but the From: field.

Thanks in advance.

Christian