I have a site running on CentOS 5.4 through Amazon EC2. Unfortunately, Amazon has had nothing but trouble with their entire IP range getting black listed from spam tracking services regularly. I need a mail server, so I setup an smtp server elsewhere that I want to send to, but I can't just send directly to it through PHP because the direct smtp request is way too slow. What I want to do is relay through sendmail, but I've never used sendmail before, so I have no idea how to configure it. All I want is for all emails sent from localhost to be relayed to one specific external server, but I don't know how to do that. I tried to find a tutorial online, but couldn't find anything that was particularly clear as to how I go about doing that. Can anyone help me out?
The sendmail feature you need to configure is mailertable. Here are simple config instructions after a base install of Sendmail.
Mailertable: http://www.cyberciti.biz/faq/force-sendmail-to-route-mail-to-specific-hosts-or-mailserver/ Sendmail full Monty minus mailertable http://www.howtoforge.com/squirrelmail-configuration-easy-steps-squirrelmail-sendmail-apache-redhat-centos-fedora
If this doesn't work for you I'd recommend using installing webmin to configure the install. I would I highly recommend configure your firewall for outbound email.
Never used them but they do have a free plan you can try out http://sendgrid.com/. My understanding is that it was specifically create to solve your problem.
As long as you are sending transactional emails, look at this SMTP Relay Service. No limit on emails that can be sent and Free option is available.