What is storage in the plans?

I got 5$/year plan. It has 2GB storage. This is my first mail server. So It might be a noob question.

What things fill this space up? Email attachments? recieved emails?
If I send email, does that also fill storage?
What happens if I use all my storage?

From what I can see it works just like with any other email service. The space is simply used by received/sent emails (including attachments).

Nobody knows? anyone?

Nope, nothing is deleted automatically. You need to do it manually.

Thanks. So sent emails also takes space. What happens if my space is full? Does it delete old items automatically? Or do I have to do it manually?

Emails, indexes, password files, alias files, sieve filters, small databases if you use Crossbox (mail.mxlogin.com), etc.