Do I use smtp, smtp-auth, smtp-auth-tls? Which port is it running on (25, 587...)? Can I use my own smtp server? Do I "have" to use my ISP's smtp server? Is my ISP blocking the receiving smtp server? Is the receiving smtp server blocking my ISP? Is their too much load on the smtp server due to spam, delaying my email? Is my IP on an rbl list? Is the ISP's smtp server on an rbl list? Did the recipients server wide spam filters catch my email? Did the recipients own client spam filter filter out the email? Did the reply get filtered out by my spam filters? If it has an attachment, did the email get falsely identified as virus/trojan infected?
Perhaps I should call them and see if they got my email!