This is something you never want to see - your email being blocked! It never got to the users in-box, nor spam folder, and if they’ve never setup Barracuda (more below), they have no idea the email was sent.
This is some aggressive filtering.
how-to-not-get-caught-in-spam-filters-with-a-new-domain is my article documenting sending transactional emails. And Barracuda has been the most problematic.
TL;DR - I fixed this problem mainly by having better content in the email.
I’ve found I can send an HTML email fine to that same address with no link in it through Gmail GUI.
Here is the offending problematic email (with a different To address)
So from reading the above it looks as if the message was rejected probably due to a local policy.
Interestingly Gmail wasn’t blocked with the same email.
Email Scanning / Barracuda
Looking at my logs I discovered some unknown IP’s hitting the server directly after the email was sent. They came from https://www.barracuda.com/.
I assume that
essex.ac.uk use this service. https://www.essex.ac.uk/staff/it-services/email-quarantine yes they do. It hits Barracuda first. It wont notify the user unless they have setup a rule (and logged in to barracuda).
https://www.barracudacentral.org/lookups/lookup-reputation tool to check reputation, and to help classify.
184.108.40.206 - - [15/Oct/2021:12:36:00 +0000] "GET /account/email-address-confirmation/fd788c4e-ccf4-4db3-b066-6354c9aff602 HTTP/1.0" 200 3318 "-" "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729)"
https://ipinfo.io/ - the tool I used to get information on an IP
I used to have a one click email confirmation GET link like:
which was doing a 302 direct to https (the link has since been fixed)
I can only assume that this triggered a rule, and didn’t allow the email to be delivered.
Well even after fixing the 302 redirect it still doesn’t work
https://wordtothewise.com/2013/07/barracuda-filters-clicking-all-links/ article talks about this, with the simple workaround of having to put a button on the form, as having an action on a GET request is not normal. As causes problems as I’ve just found here.
Have a button so the action happens on a POST.
I tried copying the email and sending from my work Gmail GUI which didn’t bounce.
This is an example of failed email bounce/block in the Gmail GUI.
So perhaps Barracuda uses the reputation of the SMTP provider to allow emails to get through.
Monitor your failures in email sending.
Watch out for the Barracuda!
Follow the advice in how-to-not-get-caught-in-spam-filters-with-a-new-domain. Which simply says to setup your email seding properly and to have a professional emails.