If you are running services and P6 on the same server, you should keep the defaults. If you want to apply the service immediately, you should set the Interval low. Remember that if the server is actively applying the service, all users will experience delays in the application.
To set your mail service settings:
Open the P6 Administrator application.
In the Configurations tab, expand your configuration.
Expand Services/Mail Service.
In the Email Server field, enter the hostname or IP address of the e-mail notification server.
In the Security field, select SSL, TLS, or None based on the security you are using.
In the SMTP(S) Port field, enter the TCP/IP port of the outgoing SMTP server.
The default is 25.
Enter anything from 1-65535.
In the Bounce Email Address field, enter the email address where you want bounced emails to be sent.
Note: Bounced emails will be sent to the person who sent them automatically. This bounce field allows you to send bounced emails to your administrators so they can investigate potential problems with emails.
In the Send Interval field, enter how often to send queued mail messages.
The default is 1m.
Enter anything from 0-24d20h31m23s647.
In the Maximum Queue Length field, enter the maximum size of the mail message queue.
The default is 250.
Enter anything from 0-2147483647.
In the Authorized User Name field, enter the name of the account to use to send mail from this mail server.
In the Authorized User password field, enter the password of the account used to send mail from this mail server.