Hi I'm a new player to D&D. I was wondering why this spell isn't available for Sorcerers? 

My question is: If one person knows the other, yet the receiver no longer knows the sender (rather it be amnesia or other magical memory interference) does the spell fail or would the person still hear it?

I understand that in Critical Role, Laura/Jester was able to cast Sending to people she had never met; yet I am personally unsure if that is how the spell works and Matthew/DM was just being very kind.

Also seems like a BUSTED way to be like "Oh this one character seems to be dead? Let's cast Sending and see what happens!"

Quote from neverwinterbill >>

My question is: If one person knows the other, yet the receiver no longer knows the sender (rather it be amnesia or other magical memory interference) does the spell fail or would the person still hear it?

I understand that in Critical Role, Laura/Jester was able to cast Sending to people she had never met; yet I am personally unsure if that is how the spell works and Matthew/DM was just being very kind.

Also seems like a BUSTED way to be like "Oh this one character seems to be dead? Let's cast Sending and see what happens!"

The spell requires only that the caster be familiar with the target.  The level of "familiarity" required should be adjudicated by the DM... but in my opinion, it can come from a personal meeting, some careful spying/scrying, or possibly, even just hearing detailed stories about them.  The only ways the spell would fail, as I understand it, is if you try to cast it upon someone who is dead, or is not a "creature" (for example, it might not work to speak with a deity, depending on your DM).  Of course, there's no concrete way of knowing whether or not the Sending "stuck," unless you get some level of reply; likewise, there is no requirement to respond to a Sending spell, even reflexively.  That's entirely at the discretion of whomever the target is.

As for the recipient, it doesn't matter what the target knows (or doesn't know) about the caster, for the spell to succeed.  If the target doesn't know them, or has been made to forget, then the source of the Sending is impossible to identify.  If the target DOES know the caster, then the target recognizes who cast Sending; no attempt to disguise themselves can hide who's casting the spell.

Quote from WingPegasus2000 >>

Is Sending basically the message cantrip, but can cast it at greater lengths?

 Sort of, yes, except a lot better. Sending has the benefit that the receiving creature can understand your message if it has an Intelligence score of at least 1 (i.e., virtually every humanoid creature and most beasts, among others); whereas with Message, the receiving creature will only hear the message and be able to reply, but it may not understand the message if it doesn't speak your language. Similarly, if they reply in a language that you do not speak, you will be able to hear it but may not understand what they are saying.  Additionally, when you use Message, the target hears the message as though it were clearly but quietly whispered to them, and can reply in kind (so that you hear their reply as a whisper, if the choose to do so). This means that if the target is deafened, they will not be able to hear your message; and likewise, if you are deafened, you will not be able to hear their reply.

With Sending, however, the message is heard in their mind, and their reply (if any) is heard in yours, so even if one or both of you is deafened, or you share no common language, you can still communicate telepathically (albeit briefly, just enough to send the message and receive a reply). In a way, it's somewhat like a nearly-infinite-range single-message use of Telepathic Bond.

While it does normally require a spell slot, there are limited ways to make this repeatable, such as as the Far Scribe Eldritch Invocation.

sends it to the persons ghost or their soul or something

the spell has verbal components so the caster at least has to be able to breath

I annoyed the heck out of my players by using this repeatedly...  They were minding their own business when one of them would get a ping inside their head and hear...

"Hello.  We have been trying to reach you about your horse's extended warranty..."

Strategically this is incredibly useful considering the era some games are liable to be placed in. Futuristic games can still have a good use of this spell if there's no other way of contacting a certain person.

I think, in a way, thats the point

Rename the spell to Mind Spam!!!!  MMwhahahahaha try and stop me!!!!

Fun fact: The total number of words for the first verse of "Baby Shark" comes to 26. Somehow, not finishing that last word makes it even *worse* than just hearing it.

DMs, please consider allowing for a nonzero amount of psychic damage for such a use of Sending.

Do we think a letter is enough to be “familiar” with someone? Example: a character sends a letter to an an NPC who has sending, could the NPC message them?

Would you say that the Ring of Mind Shielding can block this spell?  

Quote from kenziesasnake >>

Hi I'm a new player to D&D. I was wondering why this spell isn't available for Sorcerers?

It's a little arbitrary, but Sorcerers use wild magic, so some spells need finesse and planning aren't available to them. They also don't have wizard's talent for mental magic. You can read a bit more about who gets which spell here: https://www.dandwiki.com/wiki/Spell_Design_(5e_Guideline)#Class_identity.

Wait... So you're telling me that, if I cast this, there will never not be a 95% chance of it simply happening? That sounds pretty crazy to me. Oh, and all you need is a copper wire. Great, for like 1 sp any bard, cleric or wizard can send a twenty five word or less message to anyone, so long as they're familiar with that person. Like, if you were to cast imprisonment and throw someone in a warded demiplane, they could just cast this to talk to them, with only a 5% chance of it not working, so long as they know who they are fondly? Any cleric could talk to their god, any wizard could talk to demons, angels, gods and whoever else, and bards could sext anyone they're familiar with? Powerful, powerful. 

I wonder if this spell can be casted in a higher level, and then my wizard evocator can send more than just 25 words, or select a secondary target.

I believe the Aberrant Mind tag should be added as well

I don't think it should be divination or enchantment
I mean you make a person hear something. That sounds like Illusion to me tbh.

"let me sit down for 10 minutes to send one 25 word message" if anything this should be a 2nd level spell but maybe add a small limit of like "you have to be within 5 miles of each other or something like that, sorry if the first half felt mocking or something that wasn't my intent.

I think you should be able to cast it at a higher level and add more words. That just makes sense to me.