We have a database app that emailed a daily summary, as a big one-line blob of HTML.
We saw the spurious '!' insertions in Outlook. They were not being sent by the app.
I clicked on Outlook’s “If there are problems with how this message is displayed, click here to view it in a web browser”, then in the browser right-clicked “View Source”, and I found that
two spurious “!\n ” had been inserted:
I spent quite a lot of time trying to figure this out but never did. Thankfully, this is no longer a problem for us. We changed our email system and the problem went away. This is something we had planned to do anyway, it wasn't a case of using a sledge
hammer to crack a nut.
I cannot prove but suspect it was a combination of our old email server (Icewarp) and Outlook 2013 upwards. In the time between me initially posting this question and changing our email system (around 3 years) the problem became more frequent as we upgraded
more clients to Outlook 2013/2016. It wasn't an issue for 2010 users. But at the same time, it wasn't an issue using Outlook 2010 with something like Gmail.
We still have a mixture of versions of Outlook in use from 2010 to 2016. We are now using Exchange Online and no longer have this issue.
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Last updated December 3, 2019