smtp relay and spam?

If you are using an external email server to relay your emails,
makesure to add spf record to your dns:

Example:
#Create a TXT record containing this text:
v=spf1 a:mail.yourdomain.com -all

#above meants only allow email to be sent directly from mail.yourdomain.com

if you want to use google:

v=spf1 include:aspmx.googlemail.com -all

for multiple mail server:

v=spf1 a:mail.yourdomain.com a:mail2.yourdomain.com -all

-all = exclude all others..

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How to setup qmail to forward emails to another smtp server?
sudo vi /var/qmail/control/smtproutes

:yourexternalserver.com:port# username password
#port, username and password are optional

if you want to set for specific domain:
mydomain.com:yourexternalserver.com:port# username password
:otherexternalserver.com:port# username password