If your Server has a dial-up link, its domain name should have at least one additional DNS MX record, specifying a "back-up" mail server (usually, your ISP mail server). When your Server is off-line, all messages directed to your domain(s) are sent to that back-up mail server.
The back-up mail server tries to deliver collected messages to your server. Usually, the retry period is 30 minutes, so your system should stay on-line for at least that period of time in order to receive messages from the back-up server.
To avoid this delay, the SMTP module can be configured to send the Remote Queue Starting ("ETRN") command to the back-up server. When the back-up server receives that command, it immediately starts to send the collected messages to your Server.
Send Wakeups: Use these settings to specify the address of the Back-up Server, and to specify how often the Remote Queue Starting command should be sent.
Note: the name of the back-up server should be the name of the real computer (as specified in an A-type DNS record), not a mail domain name. While your provider domain name can be provider.com, the name of the provider mail server can be something like mail.provider.com. Consult with your provider to get the exact name of your back-up server, or just examine the DNS MX records for your domain: your back-up server is specified with the MX record that has the priority next to your own Server MX Record priority.
The SMTP module wake-up activity is limited with the TCP Activity Schedule.