Apple Mail Not Sending or Receiving: What's Actually Wrong
Stuck at sign-in? Separate issue — see my Apple Mail sign-in post. This one's about Mail once you're already logged in, across Mac, iPhone, and iPad.
The check that applies everywhere: storage
Before anything device-specific, I always check iCloud storage first (Settings → [your name] → iCloud). Apple's free tier is only 5GB, shared with Photos and backups, and a full plan causes exactly this kind of vague mail failure — without the actual error ever mentioning storage. This single check has saved me more troubleshooting time than any other step on this page.
On Mac specifically
If storage isn't it, a stuck mailbox index usually is. Select the affected mailbox in Mail, then Mailbox → Rebuild from the menu bar. I've found this fixes messages that won't load or look duplicated far more reliably than removing and re-adding the account.
On iPhone or iPad specifically
Check Settings → Mail → Accounts → Fetch New Data first. Push should be on where it's supported; everything else should fetch on an interval, not "Manually." A surprising number of "not receiving" reports come down to this one setting getting flipped by accident at some point.
Sending fails on any device
Check for an oversized attachment first. If you're sending as an iCloud+ custom domain, a lapsed SPF/DKIM setup at your DNS provider is the next likely cause — this shows up as sends silently failing rather than any clear error message.
Works on one device, not another
Double-check both devices are actually signed into the same Apple ID. Sounds obvious, but a second device on a different Apple ID — a family member's, or an old one — simply won't show the same mailbox, and nothing warns you about it.