How do you send email verification code to your users?

As I'm developing a social media app, I need to verify my users by sending them a verification code via email. After researching various options, I chose to use AWS SES due to their low cost and relatively low development effort. However, just before our launch date, my production access request got denied, and I felt frustrated and confused.

In my request, I provided my use case, which is sending verification codes only when users request them. I also included a template of the email I planned to send, the number of emails I would send per day, and some general statements about how I would handle bounces, complaints, and unsubscriptions. However, the rejection letter I received from AWS was vague, and they stated that they couldn't provide specific details for security purposes.

Upon further investigation, I realized that sending emails is more complicated than I initially thought. For example, preventing bounces is essential, and this requires using a third-party service that verifies whether an email address exists. However, this won't completely prevent bounces. I believe AWS SES rejected my request because they thought I lacked sufficient knowledge and didn't want me to contaminate their IP pool.

However, my concern is that there are more traps than just bounces when it comes to sending emails. I couldn't find any useful resources that could teach me how to do it correctly and which service is the best. Therefore, I'm seeking advice from the community.

What email sending service do you use, and what steps do you take to ensure email delivery success while minimizing bounces and complaints? Any advice would be much appreciated!