Urgent: Ongoing HubSpot Email Delivery Issue – Looking for Root Cause / Microsoft 365 & Exchange Gui

Smart CRM & Sales Hub
1

Urgent: Ongoing HubSpot Email Delivery Issue – Looking for Root Cause / Microsoft 365 & Exchange Guidance

Hi everyone,

We are looking for some guidance from the HubSpot community and, ideally, someone from HubSpot Support who has dealt with a similar email delivery issue.

We have been troubleshooting an email issue for several months where certain connected mailboxes are unable to reliably send and receive emails.

For example, emails sent to one of our affected mailboxes are being rejected/bounced. We are also seeing situations where users cannot send emails from the affected mailbox. Importantly, this is not limited to an existing or oversized email thread — we have reproduced the issue with completely new email threads as well.

What we have already investigated

We have worked with both HubSpot Support and our Microsoft 365/Exchange administrators.

The SMTP/NDR information appears to indicate a Microsoft Exchange/content conversion rejection. HubSpot’s previous investigation suggested that the rejection may be related to Exchange-side processing such as transport rules, security policies, signatures, or other mail-flow configuration.

However, our Microsoft 365/IT team has also reviewed their environment and does not believe the issue is originating from their side.

So currently, we have a situation where:

HubSpot: The rejection appears to be happening within Microsoft Exchange.
Microsoft 365/IT: They do not believe the issue is caused by their configuration.
Business: The affected mailboxes still cannot reliably send/receive emails.

Current workaround

We are currently having to manually work around the problem by unbouncing/reconnecting the affected email account where required and manually forwarding emails to specific users.

This may temporarily restore functionality, but it is obviously not a viable long-term solution.

We have also been advised to try starting a new email thread. However, because completely new emails are also being rejected, we do not believe this is simply a thread-size issue.

What we need

We are looking for help identifying the actual root cause and restoring normal email functionality.

Ideally, we would like:

  1. A HubSpot Support engineer assigned to this issue.

  2. A live troubleshooting call with HubSpot Support and our Microsoft 365/IT administrator.

  3. A review of the SMTP/NDR errors, message headers, timestamps and affected mailboxes.

  4. Confirmation of exactly where the rejection is occurring.

  5. A permanent solution rather than continuing with temporary workarounds.

If anyone has experienced a similar issue involving HubSpot connected inboxes, Microsoft Exchange and content-conversion/SMTP rejection errors, we would really appreciate any guidance on what ultimately resolved it.

And if a HubSpot Support representative is able to join a call with us, that would be extremely helpful. We have already spent considerable time troubleshooting this and believe a joint session would be the fastest way to get all parties looking at the same evidence.

Thanks in advance for any advice or direction.

3

Hi there @Dharmik_Jani,

Thank you for posting to the Community!

If you’d like to speak with Support, I’d recommend going to this resource. It looks like your account has access to Support since you are on a paid subscription, so I would recommend reaching out that way so that you can schedule a call with our Support team :slight_smile:

Cassie, Community Manager

4

This sounds like an issue worth investigating at the authentication and Exchange configuration level. I’d start by checking SPF, DKIM, DMARC, Microsoft 365 mail flow rules, connector settings, and message trace results to identify where the emails are being delayed or rejected.

If the issue is intermittent, comparing the headers of successfully delivered and failed messages can also help pinpoint whether the problem is HubSpot, Microsoft 365, or the recipient server :blush:

5

Thanks for the suggestions. We’ve already checked most of these areas, including SPF, DKIM, DMARC, Exchange/mail-flow rules, connectors, and the available NDR/message details.

Unfortunately, none of these checks have resolved the issue, and we’re still seeing failures even with completely new email threads.

At this stage, both our Microsoft 365/IT team and HubSpot Support have investigated, but we still don’t have a clear root cause. We’re hoping to get HubSpot Support and our Microsoft 365 admin on a live call together to review the failed messages and identify where the rejection is actually occurring.

Appreciate the suggestions!

7

A quick update on the email delivery issue we’ve been troubleshooting for several months.

After multiple rounds of investigation with both HubSpot Support and our Microsoft 365/Exchange team, we now have a clearer understanding of one of the main causes.

HubSpot confirmed that Exchange rejected emails exceeding its 250 MIME body-part limit:

554 5.3.4 – Content conversion: Maximum number of body parts (250) per message exceeded

This can occur with long-running email threads as email history, HTML/plain-text content and other elements accumulate. Once the limit is exceeded, Exchange rejects the message and HubSpot can subsequently mark the address as bounced/suppressed.

This also explains why emails may work normally from Outlook but fail when sent through HubSpot, as the two platforms can construct the email differently.

Current situation

The affected address has been unbounced in HubSpot and sending/CC functionality has been restored.

However, we are still working on the long-term solution, as repeatedly unbouncing addresses is only a temporary fix.

The Microsoft 365/Exchange administrator needs to review the 250 MIME body-part limit and relevant Exchange/transport configuration to determine whether this can be increased or otherwise addressed.

HubSpot has also recommended:

• Starting a new thread when conversations become very long.
• Avoiding forwarding lengthy email chains through HubSpot.
• Keeping email signatures simple/text-based.

We have also asked HubSpot for additional guidance on preventing the issue from recurring and requested a senior technical resource to join a troubleshooting call if possible. While a call has not yet been arranged, we remain in discussion with their senior support team.

The immediate issue is resolved, but the focus now is on finding a permanent solution rather than repeatedly applying the same workaround.

8

After multiple investigations with HubSpot Support and Microsoft 365/Exchange, HubSpot has now confirmed the main cause behind the recurring issue.

What happened

The primary error was:

554 5.3.4 – Content conversion limit(s) exceeded – maximum number of body parts (250) per message exceeded

This is a hard Microsoft Exchange Online limit. Long or complex email threads can accumulate MIME parts through quoted history, HTML/plain-text content, signatures, forwarding, etc. Once the 250-part limit is reached, Exchange rejects the email and HubSpot records the rejection as a hard bounce.

This also explains why the same mailbox can work normally from Outlook while failing through HubSpot, as the email structure generated by the two platforms can be different.

Current situation

• HubSpot has removed the bounce/suppression from the affected addresses, so sending/CC functionality has been restored.

• HubSpot confirmed that the 250 MIME-part limit cannot be increased or changed through HubSpot or Microsoft 365 configuration.

• The Microsoft 365 team can still review Mail Flow/Transport Rules, particularly rules that add signatures, disclaimers or other content that could increase the MIME structure.

• We also received a separate 550 5.2.1 – Mailbox disabled error for another address, which needs to be checked on the Microsoft 365/mailbox side.

What we need to take care of

To reduce the chances of this happening again:

• Start a new email thread when a conversation becomes lengthy.

• Avoid forwarding very long email chains.

• Remove unnecessary quoted history when replying to long-running threads.

• Where appropriate, use 1-to-1 email from the HubSpot contact record instead of continuing very long Conversations threads.

• Keep email signatures simple/text-based.

Final status

The HubSpot support ticket has now been closed, as HubSpot has provided the identified causes and recommended preventive measures.

The immediate bounces have been cleared, but the focus now is on preventing the same Exchange rejection from recurring rather than repeatedly unbouncing addresses after each incident.