Sending registration confirmation emails from VPS?

Question

Email

Hello everyone, I have a site when registering on which I should receive an email with a link to activate my account, put postfix on the VPS, sort of set it up somehow (I don’t rummage in mailers, so I followed these instructions https://www.digitalocean. com/community/tutorials/h...
Yesterday I registered on my site myself, I received an activation letter, I asked a friend for it, but it also went to the spam folder (I signed up for mail, a friend on gmail) well, actually, I asked for a couple more friends to register and they didn’t receive letters, after this news I deleted my account and registered under a different name and I didn’t receive it either, in general they stopped coming, they only come if they send a command through the console

echo "This is the body of the email" | mail -s "This is the subject line" [email protected]

Help me figure out what's wrong?
ps I ask you not to throw slippers, I'm new to this business
. I forgot to clarify, I need letters to go only in one direction.
Here is a screen that one letter came and no longer comes, only if through a command (then they come from root)

Answer 1

mailing recommendations