Nearly all webmail UIs are written using PHP. Given that 78% of PHP installs include at least one vulnerability [1] it would be good to have a webmail UI written in some other language.

[1] http://www.theregister.co.uk/2014/12/31/want_to_have_your_se...

Edit: This is not a dig at PHP, I would not want to install it on one of my servers because I do not know what I am doing.