By default all Virtual Servers are closed to SMTP relaying by the POP-before-SMTP anti-spam configuration, which is specifically designed to prevent SMTP relaying by spammers.
How It Works
Each time a user on your server successfully enters a correct username and password to your POP or IMAP server (i.e. checks an email account that is configured on your Virtual Server), the server records the IP address of the remote client. The IP address and a timestamp are stored in the ~/etc/relayers.db database file. The database serves as a list of IP addresses that are a llowed to perform an SMTP relay. A simple rule set in the check_rcpt section of the ~/etc/sendmail.cf file causes sendmail to refuse to relay e-mail from any IP address that is not listed in the ~/etc/relayers.db database file. You will need to add this Rule Set if it is missing from your ~/etc/sendmail.cf file.
You may on occasion want to manually add to or clean the relayers.db database yourself. To do this we have created a utility program named vsmtprelay that allows you to add, delete, expire, or list IP addresses in the ~/etc/relayers.db file.
Although the ~/etc/relayers.db database does not use up a lot of disk space, you may wish to automatically expire entries on a periodic basis to keep the database small. This can be done using the Cron program scheduler. For example, the following crontab entry would expire all of the day-old ~/etc/relayers.db database entries every night at 3:15 AM.
15 3 * * * /usr/local/bin/vsmtprelay expire 1440