If you're missing emails - either a specific message that someone sent and you can't find, or a broader pattern of mail not arriving - it's almost always one of a small number of causes. This goes through them in order of likelihood, starting with the things that are easiest to check, and includes specifics for Gmail and Outlook because the diagnostic paths differ.

Before anything else: is the email actually missing?

Sounds obvious, but check three places that often hide mail:

Spam / Junk folder. Both Gmail and Outlook auto-route messages they think are spam, and the bar for that has gotten more aggressive. A surprising amount of "missing" mail lives in Spam.
Other tabs (Gmail). Gmail's tabbed inbox - Promotions, Social, Updates, Forums - is a frequent culprit. A legitimate password reset that lands in Promotions looks identical to a missing email.
Archive / All Mail. If you have rules that auto-archive, the message may have skipped the inbox but be findable in All Mail (Gmail) or the equivalent folder in Outlook. Search by the sender's domain and see.

The 5 most common real causes

1. Storage full

If your Gmail or Outlook account is over quota, incoming messages bounce back to the sender as undeliverable. The sender gets a delivery failure notification; you get silence.

Check on Gmail: the storage indicator is at the bottom of the left sidebar (also visible at one.google.com/storage). If you're at or near 100%, that's your answer. Clear large attachments, empty Spam and Trash, or upgrade.

Check on Outlook: File → Info → Mailbox Settings (desktop), or Settings → General → Storage (web). Same principle.

2. A filter is silently archiving or deleting it

Filters / rules apply automatically and can route mail past the inbox. If you set one up six months ago, you've almost certainly forgotten about it.

Check on Gmail: Settings (gear icon) → See all settings → Filters and blocked addresses. Look for any filter that contains "Skip the Inbox", "Delete it", or "Forward to", and check whether it matches the sender you're missing.

Check on Outlook: Settings → Mail → Rules. Disable any rule that seems suspicious and see if mail starts landing.

3. The sender is on a block list

If you (or a previous IT admin) blocked a sender or their domain, their mail goes to Spam or is dropped silently.

Gmail: Settings → Filters and blocked addresses → scroll down to "The following email addresses are blocked". Unblock if needed.

Outlook: Settings → Mail → Junk email → Blocked senders and domains. Remove anything that shouldn't be there.

4. Your forwarding is broken

Sometimes mail arrives at the original address but gets auto-forwarded to another inbox you're no longer using - or the forwarding loop dropped it.

Gmail: Settings → Forwarding and POP/IMAP. Check that the forwarding address (if any) is correct.

Outlook: Settings → Mail → Forwarding.

5. The sender's mail is being silently dropped at the provider level

Gmail and Outlook do reputation-based filtering on the SENDER's domain. If a domain's reputation is bad enough (a startup that recently moved domains, a freelancer using a shared host), the receiving server can drop mail before it ever reaches Spam.

This is the hardest one to diagnose because both ends see success: the sender's server says it delivered, and the receiving side has no record. The only fix is to ask the sender to check their domain reputation (DKIM, SPF, DMARC - their IT person will know what to do).

Gmail-specific causes

Gmail tabs reclassification: Even if you've turned tabs off, Gmail's back-end classification still decides where things go. Search by sender to find recent mail; if it's in Promotions or Updates, drag it to Primary - Gmail learns.
IMAP sync stuck (desktop client): If you're using Apple Mail, Outlook, or Thunderbird and mail isn't syncing, remove and re-add the Gmail account. The OAuth token may have expired silently.
Gmail Push notifications disabled: Mail arrives but no notification - check Settings → Notifications and your phone's OS-level notification settings for the Gmail app.

Outlook-specific causes

Cached mode out of sync (desktop): File → Account Settings → pick your account → Change → uncheck and re-check "Use Cached Exchange Mode". Restart Outlook.
Focused inbox hiding mail: Outlook's Focused / Other split shows you a subset by default. The missing mail may be in Other. Try the "Show Focused Inbox" toggle to compare.
Clutter folder (older accounts): Pre-Focused Inbox accounts had a Clutter folder that the new UI hides. Check it via the web app.
Tenant-level mail flow rules: If you're on a company Microsoft 365, your IT admin may have set up transport rules that drop or redirect mail. You'll need their help to check.

If you're missing one specific email

Have the sender:

Resend with a different subject (sometimes a clean retry works).
Check their "Sent" folder to confirm it was actually sent.
Look at the delivery status in their mail provider - some (Gmail Workspace admins, Outlook admins) can see whether the receiving server accepted it.

If all three say "sent and accepted" but you still don't have it, the most likely culprit is a filter or Spam routing on your end.

When to escalate

If you've gone through this list and mail is still disappearing, contact your provider:

Personal Gmail: support.google.com/mail
Workspace Gmail: your Workspace admin can see delivery logs they can run to trace specific messages.
Outlook.com personal: support.microsoft.com/outlook
Microsoft 365 work: your IT admin has message trace tools that show exactly where any specific message went.

What an AI email assistant can't fix

AI assistants like Inboxer help with what gets done with your mail once it's in your inbox - triage, drafts, follow-ups. They can't intervene before delivery. If mail isn't reaching your inbox, no AI layer fixes that - the issue is at the provider level and the list above is the right place to start.

Once your inbox is reliable again, an AI triage layer makes a big difference. The reason missing-email anxiety is so common isn't usually that mail genuinely disappears - it's that important messages get buried under 200 newsletters and notifications. A tool that auto-archives the 80% that's noise means the 20% that matters can't hide. That's a different problem from this one - but it's the natural next step once delivery is back to normal.