Windows Plesk webmail shows 404 error

Check whether the domain contains webmail virtual directory with " A directory located on this computer" as "C:\Inetpub\vhosts\webmail\horde"

If it is existing do the following to fix the issue.

cd %plesk_bin%
websrvmng.exe --reconfigure-webmail
defpackagemng.exe --fix --type=webmail


This will fix te issue.