Yahoo Mail Not Sending or Receiving: What's Actually Wrong
Stuck at the sign-in screen? That's covered separately in my Yahoo login post. This one's for once you're already in and something's still not right.
Start with yahoo.com directly
Sign in on the website before touching any app. Broken there too, it's an account issue. Fine there but broken in one app, the fix is entirely local to that app.
Mail won't send
Check the Outbox first — an oversized attachment is the usual culprit in my experience. If you're sending through a third-party app, remember Yahoo wants an app password once two-step verification is on. Using your regular password there just fails quietly, which is exactly what makes it so easy to miss.
Mail isn't arriving
Spam, always, first. Yahoo's filtering leans aggressive, and checking there alone resolves more of these than every other step combined. After that, look for an old filter or forwarding rule you forgot you set up.
Both directions, all at once
Usually storage. A full mailbox can quietly block outgoing mail as well as incoming — check your usage, and empty Trash and Spam too, since those still count against your quota until they're cleared.
Comes and goes unpredictably
That pattern points to your connection more than the account itself. Try the same thing on a different network before assuming your Yahoo settings are actually the problem.