How does php implement one-site email login?

How does php implement one-site email login? The internal site already has user data. you can also create an internal "email" system. However, considering the possible communication with the external network, you still want to support smtp and pop3.
But does php mail server support one-site login?
(That is, the internal site is logged in, the email server is also automatically logged in, the internal site changes the user, the mail server also changes the user [password, disable class one ])
In addition, can I control the storage of emails (including attachments) through self-woven modules?
After reading it, it seems that there is an hMailServer. can I implement the functions I need? (I can write the code myself. I don't know any interfaces or good access ......)

Reply to discussion (solution)

Can hMailServer be in your own server? Is built on? Server? Function, if yes ?? ? You only need your php program and hMailServer to do one? Integration is enough ?? Features you want!

I have just installed hmailserver. after reading the database structure, I found that the problem should not be serious. However, I found another problem. it seems that the hmailserver can be configured with different front-end webmail. does this indicate that all front-end webmail actually communicates with the smtp/pop server through pop3 and smtp?
Because I am confused about the storage function of the mail server and want to write the storage by myself