Your email address is your unique identifier in Bitkub. It's not possible to copy or use it in another account.
Sometimes it happens that an email address becomes deactivated, it wouldn't be possible to access it. It means that withdrawing funds or cryptocurrency from Bitkub account would also be unavailable because each withdrawal is being confirmed via email.
How to prevent it
It's important to keep track of your email address on a regular basis.
Don't leave it unattended, some email providers could deactivate unused addresses and issue them once again. It's a good idea to use trusted and proven email providers like Gmail, Hotmail, etc.
Also, it's vital to pay attention to email security. Since it's the key to your Bitkub account, security measures should be taken. Protect your account with 2FA, change your password on a regular basis.
How to deal in case it happens
Contact your email provider support team. Usually lost email account can be restored