Of the PGP mails I get, most are just encrypted to me. That is cryptographically not a sound idea, but it also means that I cannot encrypt back, because I don't know which key to encrypt to.
The keyservers do not validate anything. They are just a storage medium. In fact, for my personal e-mail address, a prankster has uploaded a rogue key. There is no process by which I can ask for its removal. People who just take the key from the keyserver have a 50% chance to send me data I cannot decrypt.
This is not a PGP weakness. It has been quipped that cryptography is a method to reduce a generic problem to a key exchange problem. There is some truth to that.
It's interesting that even Edward Snowden made this mistake of sending only encrypted email.
For more elaboration about this subject K-9 resources are quite good:
https://k9mail.github.io/2017/01/30/OpenPGP-Considerations-P...
This is solved using WKD. Thunderbird already supports this with enigmail enabled
For other people there is also https://autocrypt.org/