Why do applications from the site still end up in spam?

Question

gmail

It turns out that a lot of applications come to the site, and letters began to fall into spam.
First of all, I took and excluded the letters that fell into spam from the site and everything seemed to work out, but after a while they began to fall into spam again.
Here is my pshp code for sending applications
Here is what the letter showed me Soon I

need to send traffic to the site, but here is such a problem, help guys, how to solve it

Answer 1

1. Check if the site address is on spam lists
2. Whitelist Site .xxx addresses

Answer 2

Options:

1. Set up your SMTP, which allows you to check the possibility of sending a letter to your addresses, and for it SPF, DKIM, DMARC
   
2. Use SMTP servers, for example, Yandex, if there are mailing lists, if there are not many of them
   
3. Use mailing services like Mailgun. There are free plans for small volumes.