Iphone outlook outgoing emails

Hi Dee,

If Outlook on your iPhone can receive emails but won’t send due to a full mailbox, the issue is server storage, not the app. Even with iPhone space, the server may have hit its quota, preventing outgoing messages.

To fix this:

1. Delete or archive old emails from your inbox, Sent Items, and large folders, especially those with big attachments.
2. Empty the Deleted Items or Trash folder, as these messages often count toward your mailbox limit.
3. If your account is through Exchange, Office 365, or Outlook.com, check mailbox usage via webmail: sign in at outlook.com or your organization’s web portal and go to Settings → View all Outlook settings → General → Storage to see what’s taking up space.
4. Restart the Outlook app and try sending again.

Mailbox limits are enforced by the server, so freeing up server-side space is the only way to restore sending functionality.

On iPhone Outlook, the “mailbox is full” error when sending is typically caused by either:

1. The message (usually with attachments) exceeding the allowed size limit.
2. The mailbox or cloud storage actually being full.

Use these checks and fixes:

1. Check attachment size
   • Large attachments can block sending. Many providers block messages over 20–25 MB.
   • In Outlook on the iPhone, open the message stuck in the Outbox, remove large attachments, and try sending again.
   • If needed, reduce the file size or send via a cloud link instead of attaching the full file. See also the guidance in “Do your messages contain large attachments?” which recommends keeping attachments under about 20 MB.
2. Verify Outlook.com / Microsoft account storage (if using Outlook.com/Hotmail/Live/MSN)
   • If Microsoft cloud storage is full, sending and receiving email stops. Messages sent to the account while storage is full are bounced and unrecoverable.
   • Check mailbox storage using the Outlook.com storage page (see “Check your mailbox storage” in the Outlook.com article). Free up space by deleting large or old messages, especially those with attachments, and emptying Deleted Items.
3. If using Exchange (work/school account)
   • A “mailbox is full” warning can appear even if the visible mailbox looks under quota because the hidden Recoverable Items folder is not counted in what is shown to the user.
   • Contact the organization’s Exchange/IT admin to check the actual mailbox size and quota and to clear or increase the Recoverable Items/archiving as needed.
4. If using the built‑in iOS Mail app with an Exchange server
   • There is a known issue where sending very large attachments from iPhone/iPad Mail to Exchange Server 2016/2019 (after specific cumulative updates) can show errors like “Cannot Send Mail – Your mailbox is full” even though the problem is on the client side.
   • This is identified as a client-side issue; the fix is to contact Apple Support for the iOS Mail client. In the meantime, avoid very large attachments or send via Outlook for iOS instead of the native Mail app.

If, after reducing attachment size and confirming that storage is not full, the error persists only on the iPhone while other devices send successfully, remove and re-add the account in Outlook for iOS and, if still failing, contact Apple Support or the email provider’s support with the exact error text.

References:

• I can't send or receive messages in Outlook
• Can't send or receive email in Outlook.com
• "Mailbox is full" warning even if the mailbox hasn't reached the storage limit
• "Cannot Send Mail - Your mailbox is full" error when you use iPhone mail to send very large attachments (KB5004622)