To use CritSend's SMTP relay, your main.cf file should include:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:YourCritSendUsername:YourCritSendPassword
smtp_sasl_security_options = noanonymous
header_size_limit = 4096000
relayhost = [smtp.critsend.com]:25
Note that you may need to install "libsasl2-modules"