Hello Yogiotis,
It seems like you're experiencing an issue setting up an email account on your iPhone, correct? We'd like to help.
Just to confirm, are you trying to set up that email account automatically? If so, do you experience the same issue if you try to set it up using the "How to set up your email account manually" part of the link you came from?
If you've already confirmed that the correct password for your email account was entered, do you receive the same error after restarting your device? Restart your iPhone
If your email account has already been set up and you want to add a custom mailbox, this link has some good information: Use mailboxes to organize email on your iPhone, iPad, or iPod touch
Thanks!