Bulk campaigns get throttled when Microsoft deliberately slows or stops your send to protect its servers — usually because a mailbox sends too much data or too many messages at once. This article covers the three most common causes in Outlook, how to fix each one, and what happens when a send is throttled.
What is Email Throttling?
Throttling is when your email provider limits how many emails — or how much data — a mailbox can send in a set time. Microsoft applies these limits per mailbox to protect performance. When you exceed them, Microsoft can slow your send, stop it midway, or cut off the mailbox entirely.
Outlook Attachments in the Email Body
Attaching files directly in Outlook caps your message at 4 MB and can cause sending failures. Files added to the email body in Outlook — rather than embedded through ContactMonkey — are a common trigger.
Solution: Embed your files through the File Manager instead of attaching them directly in Outlook. File Manager attachments also let ContactMonkey track who opens them.
Microsoft Outlook Sending Limits
Microsoft sets a sending limit on every email address. When you send to large distribution lists using Individual tracking, Outlook may throttle your send or stop it entirely.
Solution: If you regularly send to large lists, ask your Customer Success Manager about SendGrid. It routes high-volume sends through a separate infrastructure — letting you reach over 10,000 recipients at significantly faster speeds — and is available for Office 365 and Gmail.
Heavy Outlook Signatures
Microsoft limits each mailbox to 150 MB of outgoing data per rolling 5-minute window. A bloated auto-signature can hit that ceiling in roughly 65 messages. At that point, Microsoft cuts off the mailbox, ContactMonkey stops the send, and an alert goes out. Because the window is rolling, a slightly heavier signature can throttle a list that previously sent without issue.
Solution: Don't use Outlook's auto-signature when sending through ContactMonkey. Build your signature into the template footer instead.
What Happens When This Occurs
ContactMonkey automatically alerts both you and your Customer Success Manager when a distribution list send fails or stops midway. You'll receive an email showing whether your campaign delivered successfully or ran into issues — so you can apply the relevant fix above and resend.