Many SMTP service providers impose limits on number of emails per time period. Please consider adding a Sleep_After_Send property to smtpQ that allows setting a sleep period (in milliseconds) after each send.
Some of the controls (Task, Http) have a SleepMs method which can be used. The documentation doesn't show Mailman has this method. I don't have access to the control at the moment to verify if it does or not.
The SMTPQ service is no longer being developed further. The source code for the SMTPQ Windows Service has been published to https://github.com/chilkatsoft/Chilkat-SMTPQ
People are free to download the source, fork it, and enhance it because it's effectively just an application that uses the Chilkat MailMan as any other C++ app might do.
Chilkat focuses on API's, and supporting a Windows Service application is outside Chilkat's focus.