There are Antivirus and Antispam modules available.
They kind of work but they are just using outdate version os available open source software Clam AV and - ummmmmmmm - I can't remember what it's called right now.
All Zentyal is in reality is a stack of open source solutions that anyone can install and configure for free - the main difference is that you surrender a TON of control.
You can literally just get a copy of any version of Ubuntu Server (I strongly suggest using an LTS version) and at the end of the setup just check the boxes for LAMP, Mail Server, SSH and I can't remember if there's an option for CA but then you get to configure EVERYTHING on your own.
After that's all done then you can install Sogo (the new super good version not the REALLY OLD version) and OpenChange.
There you go it's just like Zentyal but it's all yours - no stupid WebAdmin GUI either, you can install a really runctional CPanel from a number of different options. Once you find the one you want you keep it, if you don't like it you remove it - it's literally that simple.
Not to mention the communities for Postfix, Dovecot, SpamAssasin, ClamAV, Sogo, OpenChange, Apache2 and whatever else you might land on installing are super active and you can actually get support for those products.
Did you know if you shell out the $500 for a commercial edition of Zentyal they don't support Dovecot? They also don't support Postfix... As it turns out if it's something outside of the Zentyal specific portions you don't get any help at all. They really don't even know about Samba - which is integral in their whole architecture.
That also reminds me you'll want to install Samba too - an make your own server that's up to date, rock solid and secure!