See https://bugzilla.opensuse.org/show_bug.cgi?id=1071961 for the original bug report. Right-clicking "xxx@yyy.com" and selecting "compose email" results in thunderbird being opened with "//xxx@yyy.com" in the To: field. I tracked this to commit https://git.xfce.org/apps/xfce4-terminal/commit/?id=5175d5ef5e6c9e92f4d296948528ac4bef35fe9d Simply reverting this commit fixes the issue. If there are email programs that want "mailto://xxx@yyy.com" style, then this might need to be handled in exo, not in xfce4-terminal.
The commit has been reverted, thank you! https://git.xfce.org/apps/xfce4-terminal/commit/?id=b92bc2299c01ba3364ad8518c8eb9c8d87a2dde9