I'll be talking about and showing images of Gmail, but most email programs and webmail have some form of filtering function that will be similar to what I am about to describe.
So where you see "Gmail" just insert your own email program's name instead...mostly :-)
With your free Gmail you get 15Gb of email storage.
That's a lot. It is thousands upon thousands of emails and it is easy to stop paying attention and end up filling of of that with crap just by NOT deleting anything...I mean, why would you?
Well, for one you may not fancy paying for more storage with Google. So you'll need to find a way of deleting the stuff you don't want in bulk.
CAVEAT: Back it up first. https://www.ihatemypc.com.au/post/moving-thousands-of-emails-easily
Gmail has a pretty powerful and easy to use search function.
You can specify all sorts of criteria to narrow down what you want to get rid of.
For example, I often just go to the oldest email I have and see what the date of it is. I then make a decision about how many years email I want to keep in Gmail and create a search that selects everything from (say) 3 years ago and older...and then delete the lot.
Often though, I just want to get rid of the junk and the non-essential stuff.
To this end I find an email of a type or source I want deleted, and then search on that.
For example, I get newsletters from a lot of IT sources and most aren't all that useful even after a couple of months (at most).
So I click on the email, choose "filter messages like this" and create a permanent filter on my incoming email that labels all such emails as "Newsletters".
These days I create a filter for each email I see come in that doesn't have a filter already.
Gmail's labels work kind of like folders in Outlook and other email clients but they don't move anything around. they are just a kind of "pre-search" that allows you to find so labelled things quickly.
So when I select the "Newsletter" label at the left, and then the "Select All" tick box, all 11,000+ emails are ready to be handled in whatever way I fancy (I am gonna delete them).
By creating these filters in advance of needing to tidy up, I am only a few clicks away from deleting all the stuff that doesn't need to be there.
A tidy up isn't a mammoth effort anymore, it's just how I can go about day to day maintenance (if I want) or just a matter of selecting the filter I want to purge and going for it.
Easy. Fast. Satisfying.
And, if you stuff up you still have 30 days to get things back out of your BIN (in Gmail, check you own email's rules on that).
Have at it.