you have an online mail service, you have to save email accounts of emails you receive so you can respond to those.

you allow people to forward their emails received to other email addresses, you need to save those other email addresses.

This would be in dbs for that stuff if you have third party marketing analytics, just because you have legitimate interest to save email to make application work doesn't mean you can pass that email into third party marketing analytics. That is not legitimate interest.

if you have a newsletter service and someone signs up to receive newsletter then you need to save their email to send that newsletter. you don't need to ask, they have implicitly given you permission by asking you to send them the newsletter.

If you have a process for removing users from service for violation of terms then you probably need to be able to keep information about them otherwise they can just say get rid of info and then sign on again - this would come into the parts of the Digital services acts about obligations to users and appeals process for removal etc. but different thing, if you have removed someone you need to be able to identify when they try to come on again.