[orware]

Not sure if Maddy covers this use case specifically (I didn't see it covered in the documentation, but I figure it actually is possible).

I'm hoping to use a Golang-based application as a simple SMTP Relay, which also allows for ranges of IP addresses to be whitelisted to connect openly to it from within our campus network.

Basically, the scenario is as follows: a number of internal services are older and may not support the authenticated route so we can use Sendgrid directly and send emails securely, so instead we currently have them routed to an internal Windows server which is running an older piece of IIS functionality (it's an SMTP Server in Windows that can be turned on). That piece is configured to be open (so it can receive messages on Port 25) but is also limited to a set of IP address ranges that internal servers are using (so that it's not generally usable by just anyone inside our network) and is configured to send those received messages out using Sendgrid.

I found the following Goland project just a few months back that seemed promising: https://github.com/wiggin77/mailrelay

And I'll probably need to follow up with the developer again (or try and make the contribution back to the project on my own), but the main issue it had was that the IP range whitelisting wasn't available (and even whitelisting by specific IP addresses seemed problematic, so in my testing I had to keep it completely open for it to work as expected).

If Maddy can fulfill the need instead though I can give it a shot ;-).

[foxcpp]

There is no IP whitelisting feature in maddy. I believe it is possible to do it using whatever off-the-shelf firewall you have on your system. Could you elaborate on the reasons it is not a viable way for you?

[e12e]

If whitelisting is "application level" it's easier to diagnose with an "application (protocol) level" error.

That is rather than a timeout or connection refused, an smtp 550 or 530 error with exlenation (access/relay denied; client ip not in whitelist)?

[pas]

Haraka covers this. Though it's a NodeJS SMTP server.