Why are emails not sent via php mail() (I use msmtp)?

Question

PHP

Hello!
There is a server on Ubuntu 16.04 with nginx and php 7.0 on it. on board.
I set up sending letters via Yandex SMTP, using msmtp MTA, logs are written, letters go away, but only from my PC.
There is a user without sudo rights, he cannot send letters, there is an error in the logs.
Moreover, if you connect via the sudo user and then change the user via su username, the letters again leave normally.
What could be the problem?