Not everyone receives emails via SMTP. Maybe because of the third level domain?

Question

Email

I make websites and online stores. Previously, everything was sent without problems using the mail () function, and in recent years, due to delivery problems, it began to transfer everyone to SMTP. But not everything is going smoothly with him, sometimes letters also end up in spam. Usually I send MX records of a domain to Yandex and use its Mail for Domain, or simply take yandex .ru mail.
Letters contain a few pictures, links to products and the online store itself.
I noticed that on two sites that are based on third-level domains, the problem with delivery is most common. Recipients from Mail.ru, Gmail, Rambler complain that they do not receive letters, they have to explain to them that they should look into the Spam folder. Maybe it's because of the subdomain?
And is there any universal solution, such as somehow verifying mail / domain that we are not spammers?

Answer 1

The domain level does not affect deliverability, it depends on how you comply with the standards in the formation and delivery of letters and how people react to your letters.
Check emails at https://www.mail-tester.com/ especially anything related to PTR/SPF/DKIM/DMARC
Look at your configuration and email settings
Make sure you are not being used to send form spam.