Important Note:
A wrong SPF record will affect your e-mail delivery. Only proceed if you feel confident about this subject.

SPF records are DNS TXT entries which need to be installed into the DNS server of your domain.

A good resource for constructing SPF records can be found here: http://old.openspf.org/wizard.html. Include the domain sendemails.com in your SPF record (include:sendemails.com).

Example: 

A standard SPF record which is correct for most setups (please verify before usage): 

"v=spf1 a mx include:sendemails.com -all"

The complete DNS TXT entry for bind9 DNS servers is:

@                IN    TXT    "v=spf1 a mx include:sendemails.com -all"