iMessage/ MMS issue

The functioning of iMessage is a bit confused, but here's my understanding from a few years of use:

- In a 1-2-1 iMessage conversation with 'Send as SMS' set to OFF, your iMessage will never send as either SMS or MMS.

Of course, you can still send SMS/MMS to non-iOS users, which includes the iOS 10 Messages effects which your carrier might class as MMS, so be careful who you're sending what to. (Three - one of the large carriers in the UK - even considers a rich link to a website to be MMS if it's not in an iMessage!)

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In a group conversation, your messages might be classed as MMS rather than iMessages, depending on who you're chatting with. This happens if some people in the group chat aren't using iMessage. If you're not sure everyone in a group is an iMessage user, you're safest to turn off the MMS Messaging option too.

See Apple's explainer of how group messaging works when some participants aren't using iMessage:

https://support.apple.com/en-us/HT202724

If you disable MMS, you cannot send or receive pictures except by iMessage.

MMS is either on or off. You can't turn sending off and leave receiving on. Additionally, most carriers charge for both sending and receiving.

If you turn off the "Send as SMS" option, the phone will not send messages as SMS if iMessages fail. It will not block you from sending or receiving MMSs at all. For that, you need to go to Settings>Messages>MMS Messaging>Off

Turning off MMS will not prevent you from sending or receiving pictures over iMessage. However, if the person does not have iMessage or iMessage is not working, than having MMS turned of will prevent pictures from being sent or received.