Sometimes it is a little bit confusing what password is for what purpose.

If you sign up for a web hosting package with us you will get only on account credentials: a username and a password to log in in the web space administration panel from Plesk.

In that panel you can configure everything that is necessary for your new domain, web space, subdomains, email and so on.

The login is at https://clients.ibscc.net

If you set up there an email account you can choose everything before the at sign like my-name@mydomain.tld
While creating you will be ask for a new password for that mailbox and now you have another pair of username (my-name@mydomain.tld) and password (as choosen by yourself). You need these passwords only for configuring your email client or using our webmail service.

You will find your personal webmail under https://webmail.mydomain.tld eg. https://webmail.ibscc.net 

Then you have in plesk the possibility to install just with a few clicks some 3rd party software like Wordpress, Joomla, Drupal...
If you do so you will get of course new usernames and passwords. These are only for logging in Wordpress, Joomla, Drupal... and have nothing to do with those for plesk or your email account.

All of these logins have a "password forgotten" feature. If you get lost the only thing that you have to know ist your username or email.
So the most important one is that one for your admin panel in plesk. From there you are able to change all above described usernames and password.