My workplace, about a year and a half ago, wanted to move one of the mailing lists to a forum. We did it and based it upon Simple Machines Forum. The traffic that was on the mailing list never translated to the traffic on the forum and it's been a withering shell for about a year.
To put a further nail in the coffin of our forum, a mailing list that we didn't control was stood up instead.
I'm far less likely to check a forum than my email. Email lists are also a lot easier to use on mobile devices.
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