What are the best ways to clean up my email database and to keep it healthy?

I have a huge database of over 5 million subscribers. If I do a nice, easy and small way to breakdown such as download all the contacts from Ireland, which has 7,000 contacts, how should I work out going through one by one who is an invalid contact and who is not? And to fill up the blanks such as company size or wrong typos. Is there a way to do this on excel to break up each contact? Or for every send I do for clients should I look deeply into the bounce rates and see why I had received undeliverable emails?

Any suggestions/recommendations would be hugely appreciated.

Thanks.

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source https://www.reddit.com/r/Emailmarketing/comments/hyc2vs/what_are_the_best_ways_to_clean_up_my_email/