topic Re: Email bounes that cause mail box shutdown in Email Deliverability

Email bounes that cause mail box shutdown

SBelz — Mon, 16 Feb 2026 22:19:01 GMT

Hi everyone. I have two active email campaigns and they both have high bounce back rates. When emails are sent out, they are causing in box shutdown from our sales team. What can be done to resolve the high bounce rate and stop the mailbox shutdown from our sales team?

Re: Email bounes that cause mail box shutdown

karstenkoehler — Tue, 17 Feb 2026 04:56:28 GMT

Hi @SBelz,

Could you be a bit more specific, what are you referring to exactly when you say "in box shutdown from our sales team"? Are you getting an error message? Which system is experiencing the "shutdown", the user access in HubSpot, your email service provider? How does this "shutdown" manifest?

In general, bounces can only be avoided (in bulk) by prior email validation, using tools like Zerobounce, Neverbounce, Verifalia, EmailHippo etc.

Best regards

Re: Email bounes that cause mail box shutdown

SanjayKumar — Tue, 17 Feb 2026 13:31:27 GMT

Hi @SBelz,

If you are facing high bounce rate, the first thing to look at is your data/list quality.For this, you can look into email validation tools, they basically work as a filtering layer before you send out any emails. They make sure the email you are sending to a verified mailboxes and deliverable, which reduces your bounce rates.In the long run, this reduction in bounce rate will also help you keep your mails out of spam.You may try: https://campaignkit.cc/

Another thing you could try out in parallel is email ramp up, which is basically gradually increasing your email sends per day to build mailbox reputation, reduce spam/bounce issues, and decreases the chances of getting shutdown. This practice should ideally be followed for 2-3 weeks.Suggestion: Start with a email list that have interacted with your emails in the past, this gives a positive signal.

Re: Email bounes that cause mail box shutdown

SBelz — Tue, 17 Feb 2026 17:18:09 GMT

Thank you for your feedback. I will definitely look into this these tips.

Re: Email bounes that cause mail box shutdown

SBelz — Tue, 17 Feb 2026 17:32:37 GMT

Thank you for the follow up. Attached is a screenshot from one of our sales team microsoft mailbox. The shutdown is from Microsoft and not Hubspot. I feel that our lists need to be filtered for email validation. We will look into the tools you suggested. But if you have any other suggestions how we can improve our campaign distribution, we would greatly appreciate it.

Re: Email bounes that cause mail box shutdown

karstenkoehler — Tue, 17 Feb 2026 18:07:43 GMT

@SBelz you're not going to be able to solve this with email validation -- this is something different than what you described in your initial message.

Email validation checks whether the recipient email address is valid.

This notice you're seeing is telling you that the recipient thinks you're sending spam. You can find instructions on how to confirm and solve this here: https://learn.microsoft.com/en-us/answers/questions/5576254/this-message-couldnt-be-delivered-because-the-send

Depending on the content of your emails, recipients might actually be flagging your emails as spam, leading to this notice.

Re: Email bounes that cause mail box shutdown

SBelz — Tue, 17 Feb 2026 20:29:50 GMT

Thanks for the insight! Will look into this ASAP!