Related to this I think that will be desirable if I can specify a time interval (or throttle valve) to send each email (invitation or reminder) in order to avoid spam problems.

This mail send-out could be maid from a new customer or trigered by a recent customer contact to the call center of instance

The problem with automatically sending email invitations or reminders is that someone has to trigger the process.
If you simply process such a script each time a user starts a survey, then this may leads to performance issues when having several hundred/thousand survey participants a day.

The best solution for this is a stand alone script which can be called at a certain interval by the server by using cronjobs.

Have a look at the availabe Limesurvey Cronjob Script (http://www.limesurvey-templates.com/limesurvey-cronjob-scripts-c-29.html) at the Limesurvey Template Shop: http://www.limesurvey-templates.com/index.php

This is now possible using RemoteControl2.

See
http://docs.limesurvey.org/RemoteControl+2+API&structure=English+Instructions+for+LimeSurvey#invite_participants

You can write a small program that is started every x minutes and using that function you can send out invitations.