From header is not formatted correctly:
From: "\"atul\" cus't'omer" firstname.lastname@example.org
According to RFC 2822, the address must be enclosed in angle brackets when display name is present, which means the header should look like this:
From: "\"atul\" cus't'omer" <email@example.com>.
Please note that
\" is not causing any problems - the backslash just indicates that the following character is a literal character, not a control character. The lack of
> is the problem. (See the examples in RFC 2822.)