Configure SharePoint 2013 to use external SMTP relay service with credentials

Question

Our company has an existing onsite SP2013 farm that currently uses an internal mail server for SMTP. That server is being retired and I've been given an external SMTP relay service to use instead. I have the server name, port, username & password for the new service, but so far cannot find a way to make SP2013 use it. Unlike 2019, the 2013 version doesn't give you a spot to enter credentials for outgoing mail - just a server name or IP address.

I've tried various ways of relaying the credentials with no success, but surely there's a way to do this without having to upgrade the entire farm twice (once to 2016, then again to 2019).

How can you configure an onsite SP2013 farm to use an external SMTP relay service using credentials?

---

FYI - we ended up eventually having to upgrade anyway, so the SMTP issue was ultimately resolved by upgrading to SharePoint 2019 Enterprise.

Answer 1

You can't. SharePoint 2013 can only relay to an anonymous SMTP service. You'll need to relay anonymously to an SMTP service which in turn is capable of authenticating to your external authentication-enabled SMTP service.

Answer 2

Only way I can think of doing it is less than ideal, but works. Since Windows SMTP supports authentication and relaying to another external SMTP server, you could setup SMTP on every host in the farm and have them relay through the external server. You'd have to set a host file entry on each host to make it look at itself, as localhost or 127.0.0.1 cannot be entered as SMTP servers in SP 2013 (I tried) so it has to be a hostname or IP outside of localhost it seems. The downside is you have to maintain SMTP on each server in the farm, so multiple SMTP services to deal with.

Another option would be setup a single SharePoint server and letting the other servers in the farm relay through it. I don't like this option as much though as you (and everyone else supporting SP now and in the future) has to be aware that (and remember!) it has the SMTP service on it and if you replaced that server in the farm at some point, it has a failure, or restarts, mail in the farm stops working until the server is back up or SMTP is moved elsewhere.