There are few factors to keep in mind here.
- Email is sent by 3rd Party system. You can invoke them using APIs. To send bulk email, its good if you have an API that taken multiple email id as an input. This way, you can make 1 API call to send bulk email. Please keep in mind that we are sending same email subject and body to all the email ids.
- Its good to uses some Queue management to perform this operation. This way, your microflow will not be waiting while all the emails are being sent. There are some widgets available for Queue management.
- Parallel processing can be very helpful but it has to be implemented carefully. Parallel processing can be implemented perfectly using widget or Java Handler. We used Java handler as it gives you more flexibility.
- You should have some reporting pages in Admin console to monitor all the emails that are sent. This way, you can verify if all the emails are sent.
- Mendix can be used to send bulk emails. But Mendix does not send the email by itself. Mendix can be used to design a proper solution to send email sending application. Of course you need an application to read data and pass it on to email engine. Mendix does that job perfectly.
I designed a bulk SMS and email system before where we were sending bulk SMS and email on 1 click. We used to send million emails in the matter of minutes. We were also saving reports for verification that all the emails are sent. Reporting is required because in parallel processing you need to ensure that all the threads executed successfully.
But again, it all depends how efficiently you have designed and combined various modules to create the solution.