This type of error appears in the Additional Headers field within the Mail tab when you input a line of strings in an incorrect format in a mail header field.
A correct mail header field is composed of a field name, followed by a colon (“:”), and then a field body. A field name must be made up of the printable ASCII characters (except for the colon and space).
These are valid mail header fields:
Cc: email@example.com Reply-To: firstname.lastname@example.org
These are invalid mail header fields:
Cc email@example.com // Missing colon Reply To: firstname.lastname@example.org // Space in the field name