You may not be able to prove the message, but, more to the point, it may not even matter. If you did not have an employment contract which guaranteed you employment, or specified only certain limited grounds or procedures for being terminated, you could likely be terminated at any time, for any reason. Thus, even if your supervisors received the message, they could very likely let you go.IF you had an employment contract, or very firm terms in the employment handbook (i.e. no restrictions, limitations, caveats, etc. in the handbook) which would have protected your job  if the message had been received, then in principal you could enforce those terms. But unless your communications provider can somehow find or provide proof of those messages for you, the only way to establish whether these messages were received would be file a wrongful termination lawsuit--based on breach of contract (including possibly, if the terms are strong enough, an implied contract found in the language of the employee handbook)--and then use the legal process or mechanism of "discovery" to compel your supervisors to answer questions about the receipt of the messages. Since lawsuits can be very expensive, it is not clear it would be worth doing this, especially since winning a lawsuit is never a given.