Emailing Invoices

We are emailing approximately 165 invoices from the Accounts Receivable module to our customers and have set up the Paperless Office module and gone through all the steps outlined in the instructions.  What we have found is that although many customers are receiving their invoices without any problem, some customers are not receiving their invoices at all (we have verified their email addresses to be correct in our system).  Other customers are supposed to be receiving multiple invoices and they are receiving only one (same email address but different customer numbers). We have asked our customers to check their junk mail and the undelivered invoices do not appear to be in it.  Any other thoughts on what we might do to ensure that all invoices are being delivered?