You'll need to write the code to check this

Even if zimbra did support this, then you would still need to write the code to deal with the error Zimbra would return if the name was too short, so if you are going to write code anyway then you may as well make it so that the data passed to zimbra is pre-checked.
All that said, if you dont check either, and zimbra already does not, then there's be no problem with someone making an alias of
a@company.com if they chose to, unless that's against your policy?