"get", "put", "mail from:", etc.) which the attacker can safely guess. On the other hand, if our cipher has this property (and is generally secure in other ways, too), the attacker would need to collect 264 (~1020) plaintext-ciphertext pairs to crack the cipher in this way.
The attacker can easily guess common words like "get", "put", or "mail from" if our encryption method is weak. However, if it's secure and has other strong features, cracking it with just 264 plaintext-ciphertext pairs would be extremely difficult.