How to setup mail account

You already have proof with two different mail accounts. Either you open a new post with more information about your installation, what has already been tried and with some logs. Otherwise no one here will be able to help you.

And if I understood you correctly, your Nextcloud is hosted by a third party provider, so the possibilities to do something will be rather limited anyway. Maybe it’s better to ask your hosting provider first, why it doesn’t work.