Quote from Jety_Lefr >>

It should be noted that this is a transmutation spell, and as such, it's likely that it is not considered a telepathic spell. It is a transmutation of the the air and the sound waves that travel through it.

I might be wrong, but it seems like you could interpret the spell description to mean that it functions like a cochlear device; i.e. it transmutes the sound into an electrical signal in the target's auditory nerve. This would guarantee that only the target would "hear" the message, so to speak. It's still not quite telepathy – otherwise you could think the message rather than having to whisper it. But this would explain the "transmutation" aspect.

I'm interested because I'm considering RP'ing a deaf character, and by my interpretation this spell would allow them to hear; though you'd be limited to one person at a time, and only in 6-second intervals. Nonetheless this would help them to communicate with the uhhh "non-Deaf" community. :) 

Can I combine this with the Actor feat?

Sending a message with the voice of the target for example. Or distracting enemys with the voices of their allies.

Quote from Shonisaurus >>

Question: How subtle is this? The verbal component just seems to be the message, which only the target hears. The somatic component seems to only be pointing, which could be done subtly. If I wanted to talk to a guy with whom I found a lot of treasure that we're hiding from the others in the party, could I do this unnoticed, without doing Subtle Spell?

 Relevant: https://twitter.com/JeremyECrawford/status/652550899814916096

Naturally it's up to your DM how magic works in your world, but the assumptions of RAW are that verbal components are audibly chanted mystic words, and somatic components are a complex series of gestures, and that each of those components sets the Weave in motion in the desired way to produce the spell's effect. The actual message in this case is whispered after the spell is in effect.

(Wait, how do I delete this comment. Help.)

My group is unclear about the language of "familiar with the target". How familiar do you need to be? Like, friends with? Know of existence? Have met? The biblical sense?

Quote from LunarSliver >>

My group is unclear about the language of "familiar with the target". How familiar do you need to be? Like, friends with? Know of existence? Have met? The biblical sense?

 As far as I'm aware, that's not defined in the rules anywhere, which means the term has its normal everyday meaning, and it's up to your DM to adjudicate exactly who it applies to.

Will my target find out where i am after i cast this spell on him?

Quote from FreyaHalla >>

If you use subtle spell, can you use this with no way to tell except you have to keep the copper wire on your finger or something? Could I speak into the minds of NPCs to make them think they're going crazy?

 The way my DM ruled it, when I used subtle spell I didn't have to use the verbal component to cast it, but since the spell isn't telepathic I still had to move my lips to speak the message even though only the recipient would hear what the actual words were.  Since line of sight isn't required, I'd turn away from the person and pretend to be checking on a party member so they couldn't see my lips.  (Hiding behind something or a mask would also have worked.  Possibly some uses of Disguise Self as well.)  I coupled this with the Actor feat to successfully impersonate invasive thoughts of doubt as well as successfully impersonate an NPC's patron casting a Sending spell.

Hi am now go easy on me okay dungeon master 

Pls go easy on me am serias

Quote from cojoMan >>

would it be possible to communicate with a wildshaped druid like this ?...

 They would understand you, but unless you have Speak with Animals or similar ability, you could not comprehend their reply.  This is not telepathy.

Does the receiver of the message need to have the workings of the spell explained to them before they can reply? For example, how would they be aware that any response they utter will be heard only by the caster unless the caster explicitly tells them, or does the magic somehow impart that knowledge as part of the spell?

Could this be used to 'listen in' on a snippet of a conversation by #1 targeting someone that doesn't know you (so they cannot identify you) and #2 having the message contain nothing more than a sigh or a minimally brief message like 'eh', resulting in the next few moments of the target's conversation to be also heard by you?

How long can the message be? 25 words or less as with the sending spell or can a caster press as many words in the "1 round" duration as they're able to?

Quote from quirkyfinbar >>

How can my character learn a message spell if I'm not a bard, sorcerer, wizard, or artificer? Do I need someone else to cast the spell for me?

 ...or spell tattoo, but I think that would be a one-shot.  A scroll is definitely one-shot.

Two questions:

Could this be used to eavesdrop one a target's conversation?

What would happen should one target themselves?  (LOL)

Can i use the spell to have more then one voice like a creepy echo 

yall now mardy

Can you use Message to communicate from Border Ethereal Plane to someone on Material Plane?