If there is an issue sending an email message then it will be logged to the application event log as a SMTP error. When Nintex sends an email it does so using the standard Microsoft API. If there is no error within the actual workflow or the SharePoint logs/application event log then the issue is due to the email routing and most likely an issue with your email server.
Firstly, check if the configured SMTP server address is the same as the one configured for SharePoint?
We would recommend that you temporarily install an SMTP service on one of your servers and point Nintex Workflow to that (in Nintex Workflow Management > Global Settings).
Then when Nintex Workflow sends an email, it will go into a drop folder and you can confirm that Nintex Workflow is doing what it is expected to do with regards to sending an email to the configured SMTP service. If you find that emails arrive in the drop folder, then Nintex Workflow is successfully sending emails, and the issue is most likely something to do with your “real” SMTP service.
One thing to note also is that, the ReplyTo address in the global settings page, must match the lazy approval settings email, and when user hits Reply in Outlook for LA email, it will have the global Replyto email.
I hope this helps!
This document was generated from the following discussion: General Email Troubleshooting
I would recommend going into far more detail around when the mail does NOT appear in the SMTP drop folder. That is the scenario I'm experiencing now and I'm unable to move forward.
If you are using the SMTP server provided by IIS 6.0, the default location would be here (on Windows Server 2012):
If you are having trouble with the timer service, the standard advice is to turn it off, then on again, and reset IIS. You can find it in the Windows Services control panel (AKA services.msc). It is called "SharePoint Timer Service", and it must be running for SharePoint to work properly.
I would recommend using the below tool instead of going through the trouble of setting up a local SMTP server:
SMTP4Dev is a small, free, open-source app that you can run from your user account on any machine in the network. It will start an SMTP server on your machine and show you any messages it receives in a list. In my experience it has been helpful in troubleshooting issues with Nintex email notifications.
Disclaimer: I have no affiliation with the developer of this app. And, of course, your mileage may vary.