If Mail is sitting near the top of Activity Monitor's memory list and climbing without an obvious explanation, you're seeing a specific, documented pattern that other Mac users have reported by name.
The scale of the reports
A MacRumors forum thread titled plainly "Mail App Maxing Out RAM After MacOS Upgrade" describes exactly this pattern, and a separate thread with the more colourful title "mac mail uses WAAAAAY too much memory" describes the same underlying issue. Apple's own Community forums have parallel discussions. The consistent detail across these reports is memory climbing from a normal baseline (often under 1GB) to tens of gigabytes within minutes, not hours, which points toward something actively looping or re-processing rather than a slow accumulation over a long session.
What's likely causing it
Mail's memory spikes have historically been associated with a few specific triggers: re-indexing your mailbox for Spotlight search after an update, a corrupted mailbox database file that Mail keeps trying and failing to process, or a specific email (sometimes one with unusual formatting, a very large attachment, or a malformed structure) that causes Mail to get stuck in a processing loop every time it tries to handle it. Because Mail syncs continuously in the background, a single problematic message or a broken index file can cause it to repeat the same failing operation indefinitely, which is consistent with the "minutes, not hours" timeline these reports describe.
What genuinely helps
Rebuild Mail's mailbox index. With Mail open, select the affected mailbox, then go to Mailbox › Rebuild in the menu bar. This forces Mail to regenerate its local index from scratch, which resolves cases caused by a corrupted index file specifically.
Check for the offending message. If the spike started right after receiving a specific email, particularly one with an unusual attachment or formatting, moving that message to a separate folder (without deleting it, in case you need it) and seeing if memory stabilises can identify a single-message cause.
Quit and reopen Mail. This is a genuine temporary fix; a fresh launch clears whatever state has accumulated, though if the underlying trigger (a bad index, a problematic message) is still present, the spike can recur.
Update to the latest macOS point release. Since these reports are frequently tied to a specific update, checking whether a subsequent point release has addressed the issue is worth doing before more invasive troubleshooting.
Consider IMAP re-sync as a last resort. Removing and re-adding an email account in Mail forces a full re-download and re-index from the server, which resolves persistent index corruption at the cost of needing to re-download your mail locally.
Where a memory-relief tool fits in
If Mail's spike is an active runaway loop, quitting and fixing the underlying cause (a rebuild, removing a problematic message) is the real fix; a memory tool can't stop an app that's actively re-triggering the same problem. Where Shiny is genuinely useful is the immediate aftermath: once you've quit the runaway Mail process, freeing up the memory it left behind (and any orphaned helper processes it spawned) in one click gets your Mac back to normal faster than waiting for macOS to reclaim it on its own.
For a similar built-in-app memory issue from a previous macOS version, see the macOS Sequoia memory leak guide, which covered an earlier, since-patched Mail.app leak.
Why a built-in app is more likely to be trusted, and overlooked
Because Mail comes pre-installed and most people never think to question it the way they might scrutinise a third-party app they just downloaded, a runaway Mail process can go unnoticed for longer than an equivalent issue in an app you're actively watching. It's easy to check Activity Monitor when a new app feels slow and much less common to think to check it when the culprit is something as unremarkable as your email client, which has quietly sat in the background, seemingly doing nothing unusual, for years. If your Mac has felt generally sluggish without an obvious cause, Mail is worth adding to the list of processes you check, precisely because it's the kind of app people forget to suspect.
How multiple accounts change the picture
If you have several email accounts configured in Mail (a personal iCloud account, a work account, perhaps an old account you rarely check), each one maintains its own local cache and index. A problem affecting one account's index doesn't necessarily affect the others, which means isolating which specific account is associated with the spike, by temporarily disabling accounts one at a time under Mail's account settings and watching Activity Monitor, can narrow down a rebuild to the one account that actually needs it, rather than assuming the issue is universal across every account you have set up. This kind of targeted troubleshooting is worth the extra few minutes it takes, since rebuilding every account's index unnecessarily is slower and doesn't address a problem that may genuinely be isolated to just one of them.