Switch to user Zimbra
su zimbra
Add credentials to relay_password file:
nano /opt/zimbra/conf/relay_password
Add:
smtp.sendgrid.net yourSendGridUsername:yourSendGridPassword
Apply configuration to Zimbra
postmap /opt/zimbra/conf/relay_password postmap -q smtp.sendgrid.net /opt/zimbra/conf/relay_password zmprov ms `zmhostname` zimbraMtaSmtpSaslPasswordMaps lmdb:/opt/zimbra/conf/relay_password zmprov ms `zmhostname` zimbraMtaSmtpSaslAuthEnable yes zmprov ms `zmhostname` zimbraMtaSmtpCnameOverridesServername no zmprov ms `zmhostname` zimbraMtaSmtpTlsSecurityLevel may zmprov ms `zmhostname` zimbraMtaSmtpSaslSecurityOptions noanonymous zmprov ms `zmhostname` zimbraMtaRelayHost smtp.sendgrid.net:587
Now wait for a couple of minutes and start testing it by sending out some test mails. Follow the log file to see if it works or not:
tail -f /var/log/zimbra.log