Background on the feature
“Group recipients” supports sending personalized messages to a list of tagged contacts and is a feature available from within the Webmail Add-on. This feature fills a gap between sending a traditional email message and coordinating a mass-email campaign (which is still best handled by a specialized service such as Constant Contact or MailChimp.
This feature uses your own email service which is specified under Webmail > Settings > Sending. However, email services like Google, Rackspace, etc. may detect a misuse of their system and prevent you from sending many messages. For this reason we provide an option to specify a custom outgoing mail server for delivering the group recipients messages. This override option allows you to choose a service that is purpose-designed to handle larger mail volumes, such as SendGrid, than services like Google or Rackspace are tuned for.
The system will now try to “pace” the email service, detecting and responding to “try later”, “breaking connections”, and other similar conditions. We still recommend using a purposed designed service to send bulk email, but after these changes seeing the following using a standard Gmail account is not uncommon: