Exchange 2007 and above is IMHO the best antispam capable on premises server. But I always use an incoming gateway server to ensure availability and to prevent the very obvious spam from reaching the network. SmarterMail seems decent enough for this task.
Well, if their Email is more important than their website, they should really look into outsourcing their email to Reputable Email Host with a rock solid SLA.
Outsourcing Email would be beneficial, not only in terms of security and uptime, but the addition of other features such as: backup & recovery, archiving, mobile apps, SLA, spam, etc.
Now, if its truely important that email stays inhouse, then your clients need to also look into hiring/contracting a dedicated/part time email server admin. I'm sure the cost could be justified knowing their email is more important than their website.