Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey Community Edition Version 5.6.28+230627
Own server or LimeSurvey hosting: own server
Survey theme/template: Fruity
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It seems using the "PHP" option as the Email Method works very poorly. The majority of invitations are not delivered to the participants, not even making it to their spam folders.
As we are using a Google Workspace, I have used the SMTP Relay feature as described here: (option 1)
support.google.com/a/answer/176600
and here:
support.google.com/a/answer/2956491
I created a special account for this purpose and then, while logged in to that account, visited
myaccount.google.com/apppasswords
to create an App Password.
I chose the following in the LimeSurvey email settings tab:
Email method: SMTP
SMTP host: smtp.gmail.com:587
SMTP username and password: specialaccount@somecompany.com and [app password]
SMTP encryption: Start/TLS
In the Google Workspace Admin interface I went to Apps --> Google Workspace --> Settings for Gmail --> Routing and configured the SMTP relay service as follows...
1. Allowed Senders: Only addresses in my domains
2. Authentication: Only accept mail from the specified IP addresses (and then added the IP address of my LimeSurvey server) Require SMTP Authentication
3. Encryption: Require TLS encryption
And that's what worked for me!
