Sendgrid Inbound Webhook - Undelivered notification - parsing

Question

As per sendgrid documentation, the way Inbound parser webhook responds to failures are that it will retry to post the email to the configured endpoint for 3 days and will drop the email if undelivered.
The Parse API will POST the parsed email to a URL that you specify. If a POST is unsuccessful, SendGrid automatically queues and retries any POSTs that respond with a 5XX status. This prevents data loss for customers who have misconfigured their website or POST URL.
Respond with a 2xx status to the POST request to stop the email from retrying.
To avoid returning an error your link must return a 2xx HTTP code when the email is received. This response lets our system know that your link has received the email. It is then removed from our send queue. If we do not get a valid 2xx HTTP response, our servers will believe they have failed to deliver your message. Messages that cannot be delivered after 3 days will be dropped.
I would like to know if before dropping the undelivered emails, will sendgrid notify the sender that their email could not be delivered?

Answer 1

Based on a few tests I ran, it does not appear SendGrid will ever send a notification to the sender, nor is there any easy way to ascertain whether SendGrid has dropped any inbound emails.
Although the documentation wording is slightly ambiguous, my understanding based on testing (which aligns with the documentation wording) is this:
Server returns 2xx: email is deemed accepted, no further attempts made.
Server returns 5xx: server has error, attempts made until 2xx is returned (see below for timing of subsequent attempts), or 72 hours lapses
Server returns any other response, or DNS records do not exist: email is deemed failed, no further attempts made.
My conclusions are based on a number of tests I ran over the course of a week, from which I ascertained the following:
Case 1. Parse server returns 400 or 403 error
The email is dropped after one attempt.
No further attempts are made to POST the email.
No notification is sent to the sender or to the SendGrid account.
(Test method: Configure SendGrid to a URL that returns one of the above error codes. Checked server logs over 1 week, and noted only one attempt was made.)
Case 2. The parse hook URL does not have a DNS record
The email is dropped, presumably after one attempt.
No notification is sent to the sender or to the SendGrid account.
(Test method: configure the hook URL to a subdomain that does not have any DNS records. Run DNS search and try opening the hook URL to confirm the DNS records do not point anywhere. Send an email. Then, after 12 hours, add an appropriate record to the subdomain to point it to a script. Checked server logs and confirmed no attempts were made to POST the email. A subsequent email attempt successfully POSTed.)
Case 3. Parse server returns 500 error
SendGrid attempts to POST to the hook URL at the following intervals:
+0 +5m +10m +15m +20m
+25m +35m +50m +1h20m +2h20m
...then every 3 hours
The last attempt occurs +71h20m
The 20 min offset is a bit unusual, but it falls on the hour, so it may be because the message is queued to attempt POSTing on the hour.
No further attempts are made to POST the email after 72 hours.
No notification is sent to the sender or the SendGrid account.
Statistics
The documentation refers to the availability of statistics, however, I have found the numbers to be inaccurate.
For example, during my testing, I had a number of emails go through the API, including some that were intended to be successful (and did indeed POST to my server) and a few that were intended to fail (as part of the above testing), however, the numbers returned did not align.
Takeaways
Care should be exercised in the following scenarios:
Server downtime: this would ultimately depend on how the server is configured. If the server returns a 5xx response code, SendGrid will keep trying to POST the email. I also tried to test a timeout scenario, however, SendGrid seems to be very patient (e.g. I made my script pause for 10 minutes, SendGrid kept the connection for 10 minutes, though interestingly, due to the second attempt occurring after 5 mins, the email POSTed twice).
Server misconfiguration that returns 4xx error codes. SendGrid will drop the email.
It should also be noted if an email is dropped, there doesn't appear to be any reliable method of finding this out.

Answer 2

SendGrid won't send a notification before the email is dropped. However, the message will be dropped only after receiving an error from your Webhook URL and retrying to post the message for three days.

Answer 3

We got a confirmation from the Sendgrid support team, that the emails that are dropped in the Inbound Webhook Parser feature, will not have any notification or being saved in Sendgrid. So, we won't be able to get any info regarding the emails which are dropped.