Outlook Very Slow to Send with a Large IMAP Mailbox Print

The symptom

You click Send in Outlook and the message sits in the Outbox for a long time, sometimes minutes, before it actually goes. Receiving may also lag, and Outlook spends long stretches showing that it is synchronising folders. Everything does eventually work, which is exactly why this problem drags on for months, it is slow rather than broken.

This pattern shows up when an IMAP mailbox has grown very large, several gigabytes and years of history. Webmail behaving normally for the same mailbox is the confirming clue: the mailbox itself is healthy, and the bottleneck is Outlook's synchronisation of that much mail over an encrypted connection.

Why it happens

With IMAP, Outlook maintains a synchronised copy of the entire mailbox, and the bigger the mailbox, the more work every sync cycle does. On a very large mailbox, that constant synchronisation competes with the connections Outlook needs for sending, and the encryption handshakes multiply the overhead across the many connections involved. The result is sending that waits in a queue behind sync work.

The real cause is mailbox size. The workaround below relieves the symptom, but the durable fix is making the mailbox smaller, and the workaround should be treated as buying time to do that, whether by archiving old mail locally, clearing the largest attachments, or emptying long-forgotten folders, as covered in our article on full mailboxes.

The workaround, and its trade-off

Switching the account's encryption setting from SSL/TLS to None substantially reduces the connection overhead and, in our experience with multi-gigabyte mailboxes, restores sending to normal speed.

Understand what you are trading before doing this. With encryption set to None, your mailbox password and your mail travel between your computer and the server unencrypted. On a trusted connection, your own office or home network, this is a defensible temporary measure. On public Wi-Fi, hotel networks, or shared connections, it is not, do not use this workaround on a laptop that roams onto untrusted networks. Our default advice everywhere else in this knowledgebase remains encrypted connections, and this page is the deliberate, narrow exception.

Why you must change it in the Control Panel, not in Outlook

There is a known quirk: changing the encryption setting inside Outlook's own account settings often does not stick, Outlook accepts the change, then silently reverts or ignores it. The reliable route is the classic Mail control panel, which edits the account profile directly:

1. Close Outlook completely.
2. Open the Control Panel. On Windows 10 and 11 this is the old-style Control Panel, not the modern Settings app, press the Windows key, type Control Panel, and open it from the results.
3. Find and open Mail (Microsoft Outlook). If you cannot see it, set View by to Small icons in the top right of the Control Panel window.
4. Click Email Accounts, select the affected account, and click Change.
5. Open More Settings, go to the Advanced tab, and set the encryption for the incoming (IMAP) connection to None, with the incoming port set to 143.
6. Leave the outgoing (SMTP) settings on port 587 with STARTTLS as per our email settings and ports quick reference.
7. Save your way out of the dialogs, then start Outlook and test by sending yourself a message.

The durable fix

• Shrink the mailbox. Archive old years to a local data file, delete the largest attachments (sort folders by size), and empty Trash, Sent, and Spam of anything not needed. Under a couple of gigabytes, Outlook generally behaves well with full encryption restored.
• Restore encryption once the mailbox is trimmed. Reverse the steps above, incoming back to port 993 with SSL/TLS, and confirm sending stays fast. If it does, the workaround has served its purpose and should be retired.
• If the mailbox genuinely must stay large, open a support ticket and we will look at the options for your specific setup rather than leaving the connection unencrypted indefinitely.