What is used for emailing at the backend?


I was sending emails by gmail and had no problem
now I wanted to use different email provider and cant get it going… Login denied(ec=67)
tested the settings and credentials in powershell if it works from simplest script, just cuz I was recently working with it, and it worked, so it does not seem like email provider problem
whats used at the backend to send emails? Cuz there are so many.