The Mail.Ru app, as well as the web version of Mail.Ru, allows you to use several mailboxes without entering your username and password every time you open the app.
To do this, tap the “Menu” icon in the top left corner, then tap the “Add” icon under the avatar of your mailbox.
On the next screen, select the email service where the mailbox you want to add is located. Enter your account name (username) and password, then tap “Sign In”.
Now, to switch between your mailboxes, you can simply tap on the “Menu” icon and select the avatar of the desired mailbox. The avatar of each mailbox displays the number of unread messages in it.