Community Edition Version 5.3.10+220419
Apache 2.4 running on Windows Server 2016. PHP 7.4
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I have configured LS to send invitation and confirmation emails with SMTP to smtp.gmail.com. It is working reliably, but I have a question about something I saw yesterday.  I know and understand relatively little about how SMTP works.  

When I sent about 500 invitations, I got an error from Gmail that said there had been too many logins in a short period of time.  I waited an hour or so and was able to send the rest of the invitations.

I SMTP debug mode to Always.  When I sent a batch of email reminders, I could see that the SMTP connection is closed after sending 1 email, and then immediately re-opened to send the next email. 

Is this the correct behavior? I expected it to open the SMTP connection, send multiple emails, and then close the connection.

SMTP Encryption is StartTLS.
Email batch size is 50, and the sending rate is 60.

 

afaik Limesurvey sends the emails one by one.

In any way, 500 invitations in a short period of time will cause you to run into the limits of Gmail. I don't think that it matters whether it is one SMPT connections or various.
support.google.com/mail/answer/22839?hl=...mit-for-sending-mail

I am authenticating with an account form a Google for Edu domain. Our daily limits are much higher, and within my needs.

Thanks for verifying that the emails are sent one by one. That tells me that it's not a problem with my configuration.