my Apple mail program keeps asking me to enter my password repeatedly throughout the day. Why can't it be entered for once and stay open?

I have my mail set up to receive Yahoo accouts mail via the apple mail program (stamp icon).

Even though I type in my Yahoo password for each of the 2 Yahoo accounts when I first open the program - by clicking on the stamp icon, I get a screen repeatedly asking me to enter the password.

Why do I keep getting asked to enter the password and how do I enter it only once and it stay open? In other words, why does the system keep locking me out on my own computer repeatedly?

Mac

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