I have to test a web application which sends emails to multiple users. I have to test it to send emails to more than 500 users at a time. Any tool which can create 500 users and send emails to them
There is a number of free and open source load testing tools which could stress your SMTP server, for example:
You can also consider Apache JMeter which is a multiprotocol load testing tool supporting sending emails via SMTP Sampler, it might be the most user-friendly option as JMeter is a GUI-based tool and test plan creation is pretty fast and easy. Check out Load Testing Your Email Server: How to Send and Receive E-mails with JMeter article for more details.
You might also consider locust https://locust.io/ which seems to have a lighter footprint and be awesome at generating high loads quickly. Seems the best of what I've been looking at lately..
This site a little old, but it lists various SMTP testing tools and covers POP3 and IMAP too