First check your Sent folder — if it's there, it left your account successfully and the delay is on the receiving end (or it landed in their spam). If it's NOT in Sent, it never actually sent: check your Outbox for a stuck message, usually caused by an oversized attachment or a dropped connection.

Outlook-specific fix →

This is almost always caused by having the same account added twice (once natively, once through a "add account" sync feature), or a forwarding rule that also keeps a local copy. Check your account list in Settings, and check Rules/Filters for a forward-and-keep-copy rule.

Search for large attachments first (most mail apps let you search "has:attachment larger:10MB" or sort by size) — a handful of big files usually account for most of the space. Empty Trash and Spam afterward too; deleted mail often still counts against your quota until those are emptied.

Try downloading on a different device or in a browser instead of a mail app — this narrows down whether the file itself is corrupted (fails everywhere) or it's an app-specific bug (works elsewhere). If it fails everywhere, ask the sender to resend; large or unusually named files sometimes get corrupted or blocked in transit.

Check whether the message is marked read-only or if you're looking at a shared/delegated mailbox where you have limited permissions. If it's your own account and this is new behavior, it can indicate a sync conflict — sign out and back in, or check for a pending app update.

This is typically a fetch/sync interval set too wide (check your app's "check for mail every X minutes" setting), or, less often, a delay on the sending server. If it's consistent across every sender, check your own sync settings first — that's the more common cause.

Gmail sync fix →

Make sure you're using Reply, not Forward, and check that "Conversation view" or "Threaded view" is enabled in your mail app's display settings. Some apps also break threading if the subject line gets edited.

This is usually an expired or revoked app password (common after a security review or changing your main password), or an outdated app version that no longer supports your provider's current login method. Update the app first; if that doesn't help, generate a fresh app password.

App passwords explained →

This could mean your account is compromised, or (more often, and less alarming) that your address was "spoofed" — a scammer faked the From field without ever accessing your real account. Check your Sent folder: if it's not there, your account likely wasn't breached, but it's still worth changing your password as a precaution.

Full hacked-account guide →

This usually happens when a signature is set per-device rather than synced with your account — check whether your provider offers server-side signatures (Gmail and Outlook.com both do) instead of an app-local one, so it follows you across devices.

Old regional and ISP-branded email addresses (BellSouth, SBC Global, and similar) usually still work — they've just been folded into whichever larger company acquired them, and you sign in through that company's current system instead of the original one. The address stays the same; the login page, server settings, and password recovery process change. If a bookmarked login page suddenly stops working, that's often the sign your provider quietly moved you to a new system.

BellSouth → AT&T Mail example →