[groovybits]

I was attempting to explain in a simplified manner, since OP said that they did not know much about email encryption. But if you want to be semi-technical about it:

Both the sender's and recipient's public keys are required to calculate a shared secret. That shared secret is then used to encrypt the message. The recipient's priv key is used to decrypt the message.

Edit: Validating a digital signature is typically part of the process when using all-in-one software (eg: Thunderbird's Enigmail extension). That is why I mention the use of private keys. Again, an oversimplification on my part in response to OP's statement "Is encryption on emails that I have received controlled by the sender?", which is false.

See: https://tools.ietf.org/html/rfc4880#section-2.1