Resolving smtp security breach issue on Laravel website

I have an ecommerce website built with Laravel framework that's constantly accessed by hackers and each time we change the smtp credentials they will immediately take it from the website and send spam mails. We have applied all sorts of security defence and implementations to stop them from having access to our smtp credentials but again and again they continue to take the smtp credentials to send mass mail.

I am looking for professional solutions to resolve this issue on my Laravel ecommerce website. If you have the right kind of experience, please bid and let's discuss.

Don't waste my time if you don't know how to permanently fix this issue.