Sequences causing Microsoft to Block Inbox - Enterprise Plan

[Eddie664]

I’m a new Hubspot Enterprise user. We switched CRMs from Zoho to Hubspot in January of 2026 and have been enrolling contacts into a 2-step sales sequence.

We’ve noticed a major issue. After 100 or so emails are sent from the sequence, Microsoft will block our entire email account as they think the account has been comprised from sudden bulk sending. The strange thing is, we’ve sent from this same domain for years through Zoho with no issues. I even exported all bounced Hubspot contacts and have been resending them from the same email address within Zoho the past 2 days. We have an 89% delivery rate with no mailbox blocking issue. It only appears to happen when sending through Hubspot.

As a further test, I can successfully send to the contacts 1 by 1 through my Outlook inbox and it even happened on an entirely different email addresses and on another separate domain.

I have a ticket open with Hubspot support, Microsoft support and even had both parties talk to each other on the phone. No one can seem to figure out what’s causing the issue.

I was curious if anyone else has experienced this or what I can do? At this point, it feels like I’ll be forced to switch my CRM needs back to Zoho.

[STierney]

Hey @Eddie664 - thanks so much for posting in the Community!
I’d like to start by acknowledging the frustrations I’m sure this is causing. We don’t want this to be your experience. With that said, I’d like to highlight that Sequences send emails via your connected email provider (in this case, Outlook), not through HubSpot. When you send bulk emails, you can reach email provider limits, which may be flagged even if you haven’t exceeded the number of allotted email sends per your HubSpot subscription.
Additionally, in regards to Outlook specifically, I’ve found a couple mentions of this same behavior (see here and here). Unfortunately, because these emails are sent via your email provider, HubSpot does not have jurisdiction into stopping this from occuring. My best recommendation would be to follow the outlined solutions in the Community post linked above to see if those troubleshooting steps support you in resolving this issue.
All of my best,
Shane, Senior Community Moderator

[WebGuruz_Tech]

Hi

You are definitely not alone in this; it is actually a well-documented issue between HubSpot sequences and Microsoft 365. Let me explain what’s actually happening.

Something Microsoft changed in its spam filtering is causing this. HubSpot confirmed they don’t violate Microsoft 365’s 30 messages per minute maximum, and HubSpot limits sequences to 3 emails per minute on their end. The problem is that Microsoft’s AI is flagging the sending pattern as suspicious regardless of the actual volume, likely because the sudden burst of outbound emails from a newly connected integration looks very different from your previous sending behavior.

The reason it worked fine through Zoho and even 1-by-1 through Outlook is that both of those sending patterns look completely normal to Microsoft’s filters. HubSpot sequences, even at low volumes, send in a way that Microsoft’s system doesn’t recognize as organic.

What you can do to fix this is make a custom outbound spam policy in the Microsoft Defender portal. To set up a new outbound policy and apply it to the senders who are affected, go to security.microsoft.com, then Email & Collaboration, then Policies & Rules, then Threat Policies, and finally Anti-spam.

Get your email administrator involved in this. You don’t want to change any of the settings for outbound spam without knowing what they do.

But there are a few other things that are worth checking in the meantime

First, make sure that your domain’s SPF, DKIM, and DMARC records are all set up correctly in HubSpot. HubSpot sends from a different infrastructure than Zoho, so your DNS records need to explicitly allow it. This is a very common mistake when moving CRM systems.

Second, if you have Sales Hub Enterprise, you can set a custom daily send limit in HubSpot that is higher than the default sequences limit. This lets you slow down the rate even more so that Microsoft’s filters don’t kick in. While you work this out, go to Settings → Objects → Activities → Email Frequency Controls and set a lower limit per minute.

Third, while the Defender policy change takes effect, try enrolling fewer people in your active sequence at a time, like 25 to 30 instead of 100 or more.

Hope this helps!

This is a fixable infrastructure issue, not a fundamental HubSpot problem. The combination of the custom outbound spam policy on the Microsoft side and proper DNS authentication on the HubSpot side has resolved it for others in exactly your situation. It’s worth exhausting those options before making a CRM decision based on it.

Hope this helps !