Hi roger441, is it just those 2 senders that you're aware of? What are the error messages they receive? Are they sending to you as part of a group mailer or a single individual typed out email? It's not uncommon for mailer lists to become corrupted and mail will then fail to reach intended recipients.
Please log in to My Account if you need to view or pay your bill, manage boosts and track your usage. From My Account you can also check your connection and test your line for any issues in the Service Centre.