Is it bad to send user email from your own server?

Question

Email

Good afternoon,
I just started to deal with the email server, set up postfix, screwed DKIM, and realized that I can send soap not only with the name of my domain, but also from the gmail domain, and then I had a question how critical it could be if I let it be possible to send messages from the user's email.
An example user came to my site and registered with his real mail, for example [email protected], then he wants to send something from my site and I put his real mail [email protected] in the from field.
What consequences can this entail?

Answer 1

The letters themselves will end up in spam, and then the server will be added to all spam lists.

Answer 2

sms is expensive, e-mail is not reliable (spam list). means users need to be notified through the service itself provided by them when they use it ....

Answer 3

Letters from foreign domains do not pass DMARC checks, at least mail.ru and gmail.com will show a warning on letters sent by your server from a gmail address.