Do you have, or have you ever had, a SNADS gateway installed in this site?
When a mailbox is created, it is assigned unique proxy addresses. You can view these on the e-mail addresses tab of a mailbox. Each mailbox by default gets at least an X.400 and an SMTP proxy address. By installing additional connectors into your site (such as MS Mail or SNADS), you ensure additional proxy addresses are created for new mailboxes (one new proxy address per connector).
In the Configuration, Site Addressing container you will see a list of all the currently installed proxy address types, here you have the opportunity to configure the form of the address and tick or clear a tick box to determine whether each particular proxy address is generated or not.
The actual generation of the unique proxy addresses is achieved by dll files, one per proxy type. You can see these in the Configuration, Addressing, E-Mail Address Generators container. There is usually two for each proxy address type, one for Alpha and one for i386. Clciking the properties of these entries you will see the actual dll file that is used to generate the particular proxy address.
So your exchange site thinks you have a SNADS connector, and the option to generate SNADS proxy addresses is ticked, and it is thus trying to access the SNADS proxy address generation dll (ibmpxgen.dll), and either that file is missing or corrupt, or someone has removed the definition that that file is the SNADS email generator.