If you are having problems with your mailbox, we'd first suggest checking that you are using the correct username/password by logging in to webmail at webmail.'domainname.com' with your full email address as the username.
If this reports a password error, or you would like to reset your mailbox password for security reasons, you can do so via your Plesk control panel. Please note that changing a mailbox password will require you to update it on all devices/client software that is currently using the old one.
1. Log in to Cpanel
2. Click on Account Information, List Accounts and then then the domain on which the mailbox resides.
3. Click on the Email Accounts button
4. Click on the password button of the mailbox you would like to change the password for.
5. Enter the new password into the two boxes indicated. NOTE: We strongly urge you to use a service such as Random.org Password Generator to generate a secure password for your mailbox - 12 characters/numbers minimum is recommended. We see mailboxes hacked due to insecure passwords regularly.
6. Click 'OK' to set the new password.
Note: Your server with Nimbus is capable of being used as a basic mail server, but the ongoing battle between spam management, hacking and blacklisting means that Office 365 and Google Apps are the go-to platforms if email is critical to your business/site. Read more here.