Thanks for the links, Denis. I am actually researching sample code for a feature to provide multiple SMTP servers at a single Limesurvey systems. Mainly to deliver 100.000 emails using different servers.
The idea is to have e.g. 5 different SMTP server settings and every 1000 emails the system uses a different setting for sending invitation emails.

The new LimeMailer is done for this (in fact is done to do anything to email …)

You can set whole mailer config …

But maybe there are 3.X solution ?
Check gitlab.com/SondagesPro/mailing/smtpByUser

1. You can add a counter each time a mail is send
2. You can check this counter before sending any email
3. You can set the settings according to currrent counter

DenisChenu wrote: The new LimeMailer is done for this

The new "LimeMailer" is what?

jelo wrote:

DenisChenu wrote: The new LimeMailer is done for this

The new "LimeMailer" is what?

A new class done for sending email, extend PHPMailer and updatable by plugins

See www.limesurvey.org/manual/BeforeEmail

Thanks for your valuable feedback, Denis!

I may get back to you asking for a cost estimation for implementing the multi-SMTP emailing feature.