When people with iPhones reply to someone who has switched away from Apple, and they use "Messages" (which does both iMessage and SMS) then there's a reasonable

When people with iPhones reply to someone who has switched away from Apple, and they use "Messages" (which does both iMessage and SMS) then there's a reasonable chance that the message will go into a black hole, where it will be delivered to iMessage and never appear on the other party's new phone. This is doubly true if they have ever used an Apple PC to receive iMessages; in that case it will almost certainly never reach the new phone via SMS. I don't believe there is a fix to this; sometimes you can repair it on the other party's phone by munging the contact, but most messages will just be blackholed.