On Windows it is troublesome to use message-box in order to report that password is wrong

[yasuhirokimura]

OS: 64bit Windows 10 version 1909
Emacs: 27.1 (self build with MSYS2)
Mew: HEAD of master branch of Git repository

Let's assume user mistyped password. If it is either IMAP or POP then message is displayed in minibuffer that password is wrong. On the other hand if it is SMTP then message is displayed with popped up window. It is because message-box function is used to display message. On Window the popped up window often disappears before user clicks it (for example focus accidentally moved to other window). Then following message is displayed in minibuffer.

"error in process sentinel: Quit"

And after that user can't send mail with SMTP until he restarts Emacs.