I am not sure this is a sub-issue here, but my problem is certainly related.

When I enter an ampersand in the forum post, the ampersand itself and all further text is lost in plain text emails. If there is a semicolon somewhere after the ampersand, all text after the semicolon is included in plain text emails again. For some reason, the ampersand, all letters and white spaces after it, and the semicolon are interpreted as one big html entity.