Dovecot: Disconnected, connection closed, auth failed

 From webmin dashboard>Servers>Postfix Mail Server> Virtual Domains> Find specific user maping 

How to uninstall or remove dovecot-core software from Ubuntu

sudo apt-get remove dovecot-core

sudo apt-get remove --auto-remove dovecot-core
sudo apt-get purge dovecot-core
sudo apt-get purge --auto-remove dovecot-core

Install or Uninstall Postfix from Ubuntu Server

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Installing Postfix Package

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sudo apt update

sudo apt install postfix

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Unnstalling Postfix Package

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sudo apt remove postfix

sudo apt autoclean && sudo apt autoremove

How to Install RoundCube with Postfix, Dovecot, NGINX, MariaDB (or MySQL) on Ubuuntu-20.04

wget https://github.com/roundcube/roundcubemail/releases/download/1.4.11/roundcubemail-1.4.11-complete.tar.gz

tar xzf roundcubemail-1.4.11-complete.tar.gz

ls

sudo cp -r roundcubemail-1.4.11 var/www/html/roundcube

sudo chown -R www-data.www-data var/www/html/roundcube/

sudo chmod -R 775 var/www/html/roundcube/ [temp,logs]

mysql -u admin -p

How to retry delivery of Postfix mail in queue?

 postsuper -r ALL && postqueue -f