Hi, I understand where you coming from. Amazon SES is great until you ran into issues with their compliance department.
This speed issue is a Mautic issue, because a ‘too powerful server’ will send too fast for SMS SMTP. You can mitigate it:
If you are a small sender, you can manage IP reputation, it’s starts to tricky if you need to send 2-3x a week 300k emails. Right tool for the right purpose.