How to increase the reachability of emails from a domain to recipients (gmail)?

Question

gmail

Good afternoon. There is a problem of reachability of letters to addressees. At what a problem only on Google.
I don't see such problems with yandex mail.
Most reach, but 30-40% are lost.
There is a certificate and sending through domain mail goes, mail lies on yandex.
Those letters that reach. Shows security: Standard Encryption (TLS).
Sending goes from a site using smtp. The answers come back positive while doing so.
The client is complaining. That letters do not reach. And I have no idea how to fix it.

Answer 1

If the client complains, then let him see what exactly happens with the letter. Ideally, it would be necessary to contact the client who has his own mailer, and who can see the logs.
You are sending from a public server, with which you cannot do anything special.