A bug in MailButler, a popular Apple Mail plugin, results in an empty subject line when replying to individual emails. The problem seems to affect only Exchange accounts, and there is currently no fix available. The good news is, there is a workaround.
I am a huge fan of MailButler and have been using the productivity tool and Apple Mail plugin for a couple of months. A few weeks ago, I noticed that Apple Mail started crashing more often than usual. I have been using Beta versions of macOS since Apple released macOS 10.12 Sierra. So the occasional crash of individual applications isn’t unusual.
I also noticed that when replying to individual emails, the subject line of my reply in the composer window would be empty. At first, the issue only occurred every few days and I could quickly, albeit temporarily, fix it by restarting Apple Mail.
Recently, I found myself restarting Apple Mail multiple times a day and so I spent more time narrowing down the cause.
MailButler and Exchange
To narrow down the offending component that was causing the issue, I disabled all Apple Mail plugin using a free tool from SmallCubed called Mail Plugin Manager. Without any plugins enabled, I worked with Apple Mail for a full day without experiencing any issues.
Update: The Mail Plugin Manager does not seem to be available for download anymore.
Next, I enabled Mail Act-On, one of my other productivity tools I use on a daily basis. Another day had passed without the issue occurring, so I reenabled MailButler and the problem returned.
As part of my testing, I realized that the issue would only occur with my work account. It’s an Exchange (Office 365) account, whereas my other email accounts are IMAP and didn’t experience the problem. A few days ago, I migrated two of my personal IMAP accounts to Office 365 and the ’empty subject line’ issues suddenly affected those accounts as well.
I reached out to Feingeist, the company behind MailButler and after a while, they confirmed that other customers reported the same issue. Interestingly, not all their users with Exchange accounts are affected, which appears to make finding the cause more complicated.
As of this writing, Feingeist doesn’t know what is causing the issue in Apple Mail. But they assured me, that the matter is currently being investigated with high priority.
Workaround for the ’empty subject line’ bug
The good news is that you can temporarily fix the problem by restarting Apple Mail. I hope Feingeist finds the cause and fixes the problem soon. When that happens, I will let you know by updating this article.
Update 2017-10-04: Feingeist released a new beta version of MailButler (Build 6894) that contains “some more changes related to Exchange accounts (missing subject line).”