Normally you would send email through your smtp server for your ISP/email account unless you are trying to hide your email address/identity/IP address. Many ISP's are not accepting email directly from a person to their smtp server unless they have a email account on that server. Many ISP are checking origin of email and if it is not from a registered email server(IP address) it won't relay email or accept the mail. With 80% of email today as spam ISP's are cracking down on people using their resources for someone elses gain.

So just send the email through your ISP's smtp server and all will work just fine.

HTH