mail library suddenly bloated

Not long ago -- let's say three weeks -- my internal drive had close to 100 GB free. I don't keep much work on the internal drive (I moves things to an external every day or two), since that piles up too fast, so I was very surprised yesterday when I got a notice that the hard drive was almost full. I saw that there was only a couple hundred MB free. ?? Next thing I knew, it said full. So I sorted by size, and saw that my Library had gone up to 280 GB (!!), with the Mail folder taking up 222 GB. ***?!

What would make the Mail fill up in such a such time, and, more important, what is the most intelligent way to archive the old correspondence? I really MUST archive, rather than deleting old emails.