This is a great question. We don't recommend sending lilies to anyone who owns a cat because lily pollen is highly toxic for felines. There's always a danger that cats will brush up against the pollen and lick it off their fur, which would be bad.
Azaleas and amaryllis are also highly toxic to our furry friends (if ingested) so best to avoid sending these to a pet-owning household, too.
Other plants and flowers don't have the same level of danger to our furry friends, but if you or your recipient do have pets, it's a good idea to keep your plants or flowers out of their reach just in case.