How to send corporate logins and passwords from accounts?

Question

SMTP

Good day to all! The question is this:
There is a small self-made program in C # (if necessary, I can provide the code) that generates a message that contains the username and password from the user account. I have one CSV file for ~50 users. All information is taken from the file and sent to the user's mail. At first, everything worked fine, because I used Google's SMTP server (I sent everything via gmail). But yesterday I encountered the fact that letters began to fall back into spam and users do not find these letters.

The question is, how can this be fixed? Should I use another SMTP server? Or do I need to do something with my program? I am a newbie in this business, I will be glad to any information and criticism.