I guess it depends on the context, and what a proof would consist of.
It's possible to prove that something was written at a certain time using a timestamping service or protocol. Essentially you just need someone to sign your document along with the time of the signature. There are services that do this at a not-insignificant fee, and there are also "outside-the-box" solutions that accomplish roughly the same task.
But all of these prove that something was written, not that it was sent (or more appropriately, received).
The concept of "sent" is pretty hopeless if not paired with "received", since I can send you an email, but keep it trapped on my own server for some time (and in fact modify it if desired), before eventually letting delivery happen. Likewise, your mail server can "receive" an email but not insert it into your mailbox (think spam trapping).
So, the only useful concept here has to be round-trip verification. If you send an email, and your recipient sends back a reply (preferably signed, and including the claimed signature time), then you can prove that the recipient claimed to receive your email at a given time. It's not quite the same thing, but it could be appropriate for your needs. Alternately, you could use a trusted timestamping authority to certify the time the signed receipt was received.
Some other variation on these concepts might instead be appropriate; all depending on your needs.