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://www.openspf.org/SPF_Record_Syntax
Include the domain rapidsmtp.com in your SPF record (include:rapidsmtp.com).
A standard SPF record which is correct for most setups (please verify before usage):
"v=spf1 a mx include:rapidsmtp.com -all"
The complete DNS TXT entry for bind9 DNS servers is:
@ IN TXT "v=spf1 a mx include:rapidsmtp.com -all"