Converting from MySQL

Hello,

I am trying to install a mail server using PostgreSQL on a Debian 9 Stretch.
Document I am following is for MySQL. I converted other simple SQL commands.
Failed to find PostgreSQL equivalent for below one because I did not
understand what it does in the first place.

SELECT
CONCAT(SUBSTRING_INDEX(email,'@',-1),'/',SUBSTRING_INDEX(email,'@',1),'/')
FROM users WHERE email='%s'

Any help is appreciated.

Thanks & regards,
Ertan Küçükoğlu