Having issues receiving confirmation/verification emails

Hello, we use mailsac as part of our QA to receive verification emails. Recently, we’ve noticed that our emails were not showing up in the mailsac inbox consistently. Was wondering if we were being throttled or blocked? We are on the free plan.

Time: approximately 2022-10-14 07:55:51.406
Sender: [email protected]

It’s odd since other emails from this sender are getting through.

Then a similar verification email was sent at approximately Fri Oct 14 2022 11:08:29 and showed up in the inbox. Making it appear as though there’s some text filter or throttling that is blocking the email.

Appreciate any help you can provide on this topic.

Hi SoLetsDev,

Sorry your having some trouble getting mail through.

It looks like you are sending to several different @mailsac.com email addresses. I’m not sure which timezone your timestamp was in so I’m having some trouble finding the exact email that didn’t go through.

Could you provide the email address you attempted to send mail to and the timestamp in GMT?


Hey Michael,

The email address these were being sent to was [email protected]

Ahh darn timezones, I’m in PST. So in GMT the times should be
Failed Attempt: 2022-10-14 14:55:51.406 GMT
Successful: 2022-10-14 18:08:29 GMT

Hey SoLetsDev,

Thanks for the specific time stamps and including a timestamp of a working delivery.

I didn’t see any record of any emails to [email protected] between 2022-10-14 14:50:00 GMT and 2022-10-14 15:00:00. It doesn’t seem like you are sending a quantity of email that would have triggered a throttling event.

I also checked your sending IP to see if it somehow ended up on our blocklist. There were no IPs on our blocklist within the /24 that microsoft is using to send your mail.

Can you check if Microsoft has any logging information about what happened to the message? Maybe it is stuck in a queue? Its possible delivery failed for some kind of transient reason and microsoft will retry.


Sorry for the late reply, were you able to find the email that was transmitted successfully at approximately 2022-10-14 18:08:29 GMT?

I agree with the undelivered mail, it sounds like mailsac didn’t even receive the email which indicates an earlier step might be blocking the submission. Will need to keep digging :slight_smile:

Hi SoLetsDev,

I can confirm I found the email sent on 2022-10-14 18:08:29 GMT. Our timestamps were about 3 seconds off, but close enough :slight_smile:

We did deploy an update to the mailsac SMTP server on Oct 17, 2022, at 14:08:23 GMT that should increase our observability of mail flows. If you have another message go missing, let me know and I’ll see if I can find what happened to it.


We have another one that was “sent” and not received.
SENDER: [email protected]
RECEIVER: [email protected]

Time (Approx) 2022-10-17 15:54:53.890 GMT

If this email does not show up on your side either, it gives an even stronger indication that the server we are using is preventing it from going out somehow.

I saw a couple email message delivered successfully around 2022-10-17T15:12:34.795Z with a different from address, but nothing at 15:54.

Ahh right, those emails were direct from my email as a test.

You’ve been very helpful Michael! I’m going to pursue the steps before the email hits mailsac and update this ticket when I find the cause :slight_smile:

Closing the loop on this one. The emails were being blocked by our SMTP server and did not reach Mailsac.

Once we changed the server, emails started working again.

Thank you for all your support!

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Hey SoLetsDev,

Thanks for providing the update. It is good to hear that you found the issue. SMTP delivery can be really tricky considering how many servers and layers of security are involved.