You may have noticed that when adding SMTP addresses, Active Roles (MMC or WI) allows you to include spaces. Even though Exchange does not support it.
This is by design. Active Roles does not interact with Exchange when performing such update, the changes are all made/stored directly in AD (proxyAddresses attribute). The same behaviour (spaces allowed) is noticed natively, when one alters the proxyAddresses attribute.
Deploy a PGV policy preventing the use of spaces (error displayed when trying to ok/close properties window). Note that this would affect all other types of address, as well as SMTP ones.
If spaces are allowed in your environment for other types of address, create and deploy a custom script policy that would check the 'proxyAddress' attribute for strings that start with 'SMTP:', and if so, do not allow spaces, otherwise allow spaces, etc.
An enhancement request (TF00 630843) has been created detailing the feature (prevent spaces from being allowed for SMTP addresses). The product team will evaluate the request and this feature may become available on a future release of the product.