Your SMTP server wont relay material from crap@blah.com. Most likely this is because blah.com isn't the domain the server normally handles. At least thats been the reason when ive seen this error. For instance if I send from "demerphq@lala.com" i get the error you see. If i send from "demerphq@mycompany.com" I dont.

BTW, the part of the code that determines who you are sending from/to is the $smtp->from() and $smtp->to() commands, NOT the text you sending in "To: ..." the latter stuff is strictly for humans and their mail cleints and is ignored when the mail is sent. Thats why its possible to send or receive mails without these headers at all.