If Outlook on your iPhone suddenly stops syncing (no new mail, delayed delivery, or folders not updating), you can usually narrow it down quickly by matching the symptom to a likely cause.
Start with the mobile steps first, then move to desktop steps only if the account itself needs repair.
Here’s a practical symptom → cause → fix mapping.
1. Symptom: New emails arrive only when you open Outlook
Most likely cause: iOS background limits or Outlook background refresh is off, so the app doesn’t wake to fetch.
Fix (iPhone first):
- Go to Settings → General → Background App Refresh. Turn on Background App Refresh and enable it for Outlook.
- Go to Settings → Mail → Accounts → Fetch New Data. Enable Push if available, and set Fetch to a reasonable interval (not Manual).
- Go to Settings → Battery and temporarily disable Low Power Mode.
- Open Outlook → your profile icon → Settings → tap the affected account → ensure Sync Contacts/Sync Calendars toggles aren’t stuck (toggle off/on once if they are).
If it starts syncing only on Wi‑Fi but not on cellular, jump to section 4.
2. Symptom: Outlook says “Last updated” a long time ago (or a spinning sync never finishes)
Most likely cause: a stuck sync state, corrupt local cache, or an account token that needs to be refreshed.
Fix (iPhone first):
- Force close Outlook (swipe up from the app switcher), then reopen it.
- Toggle connectivity: enable Airplane Mode for 10 seconds, then turn it off.
- In Outlook: profile icon → Settings → tap the account → tap Reset Account (this re-syncs mail without deleting the app).
- If it still won’t update: remove and re-add the account inside Outlook (profile icon → Settings → select account → Delete Account), then add it again.
Desktop step (only if re-adding fails or immediately breaks again):
- Sign in to your mailbox on desktop web (Outlook on the web / Microsoft 365). If mail is missing there too, this is server-side, not an iPhone issue.
- If web is fine, change your Microsoft account password and sign back in on iPhone (this forces a clean token refresh across apps).
A single password change can clear a surprising number of “stuck sync” loops.
3. Symptom: Some folders sync, but others don’t (or search can’t find recent mail)
Most likely cause: the account is hitting a sync scope limit, a folder is not subscribed, or the mailbox is too large for the current local index.
Fix (iPhone first):
- In Outlook, go to the folder list and pull down to refresh while you’re inside the problem folder (not just the Inbox).
- If you use IMAP: check whether the missing folder is actually subscribed (this is often easier to confirm on desktop—see below).
- In Outlook iOS settings for the account, look for any options related to sync period (for example, syncing only the last X days) and increase it if available.
Desktop step (recommended for folder issues):
- On desktop Outlook or your provider’s webmail, verify the missing folder exists and has the messages you expect.
- For IMAP accounts, check folder subscriptions/visibility in the desktop client. If the folder isn’t subscribed there, the phone often won’t pull it consistently.
If only “Sent” is wrong (messages missing or delayed), jump to section 5.
4. Symptom: Sync works on Wi‑Fi, but not on cellular data
Most likely cause: cellular data is disabled for Outlook, or iOS is restricting data in the background.
Fix (iPhone first):
- Go to Settings → Cellular and make sure Outlook is enabled.
- Go to Settings → Cellular → Cellular Data Options and ensure Low Data Mode isn’t limiting background activity on your active line.
- If you use a VPN/ad blocker profile, temporarily disable it and test sync again (some configurations block Microsoft sync endpoints).
Desktop step:
- If your account is managed by work/school, ask IT whether mobile access requires a compliance profile or whether conditional access policies recently changed.
5. Symptom: You can receive email, but sending fails (or Sent items don’t appear)
Most likely cause: SMTP/Auth mismatch for non-Microsoft providers, an Outbox message stuck, or server rules that place sent mail somewhere unexpected.
Fix (iPhone first):
- Open the Outbox (or Drafts) and remove any message stuck “sending,” then try sending a short test email.
- Remove and re-add the account in Outlook to refresh server settings and authentication.
Desktop step (to confirm what the server is doing):
- Send a test email from desktop/web and see where the server stores sent mail (Sent, Sent Items, or a provider-specific folder).
- If you’re using a custom domain/provider, confirm whether they require app-specific passwords or modern auth—older SMTP setups can fail silently on mobile.
If sending fails only on one network (e.g., office Wi‑Fi), the network may block mail ports—test on cellular to confirm.
6. Symptom: Notifications stopped, but sync is actually working
Most likely cause: notification permissions changed, Focus/Do Not Disturb is filtering alerts, or Outlook’s notification settings are off.
Fix (iPhone first):
- Go to Settings → Notifications → Outlook and enable Allow Notifications, plus Lock Screen and Banners if you want them.
- Check Focus modes (Do Not Disturb/Work/Sleep): allow Outlook notifications or disable the Focus temporarily.
- In Outlook: profile icon → Settings → Notifications and confirm the affected account and folders are enabled for alerts.
It’s possible for mail to sync fine while notifications are blocked.
7. When it’s not your iPhone: quick desktop checks that save time
Most likely cause: the mailbox itself is having issues (quota, security lock, password change, or provider outage), so iOS troubleshooting won’t stick.
Desktop step checklist:
- Sign in on the web and confirm new messages appear there.
- Check mailbox storage/quota (full mailboxes can stop accepting new mail or break sync).
- Review recent security prompts: password changes, “unusual sign-in,” or forced re-auth can pause mobile sync until you sign in again.
- If this is a work account, confirm whether a new device policy requires re-enrollment or an updated authentication method.
If webmail is also delayed, you’re likely dealing with a provider-side issue rather than Outlook on iOS.
Final thoughts
Most Outlook-on-iPhone sync problems come down to background refresh, fetch/push settings, or an authentication token that needs a clean reset.
If the same account behaves badly on iPhone but looks perfect on desktop web, removing and re-adding the account (or using “Reset Account”) is usually the fastest way to get back to stable syncing.