Is this sentence correct?

[zaffar]

Is this sentence correct?

"What Mac did was a blunder, since he forwarded sms to Ainy.

Please reply.

[Raymott]

Is this sentence correct?

"What Mac did was a blunder, since he forwarded a/the sms to Ainy.

Please reply.

It's grammatical.
If the blunder was forwarding the sms to Ainy, then I wouldn't express it this way.

[emsr2d2]

Is this sentence correct?

"What Mac did was a blunder, since he forwarded sms to Ainy.

Please reply.

I think you mean "Mac made a blunder by forwarding that SMS to Amy".

[zaffar]

Is this sentence correct?

"What Mac did was a blunder, since he forwarded a/the sms to Ainy.

Please reply.

It's grammatical.
If the blunder was forwarding the sms to Ainy, then I wouldn't express it this way.

Then what does this sentence "What Mac did was a blunder, since he forwarded the sms to Ainy" actually means, since I want to say that Mac makes a blunder by forwarding the sms to Ainy. Please reply.

[Raymott]

Then what does this sentence "What Mac did was a blunder, since he forwarded the sms to Ainy" actually means, since I want to say that Mac makes a blunder by forwarding the sms to Ainy. Please reply.

If that's what you want to say, then say it. "Mac made a blunder by forwarding the sms to Ainy." This identifies the blunder with forwarding the sms. The original doesn't.
But again, you've used 'since' wrongly in your question. 'Since' here means 'because'. Now you are asking me what one sentence means because you want to say another.
"Then what does X mean, since I want to say Y?" 'Since' doesn't make sense.
If you left out 'since' in the original, it would be right: "What Mac did was a blunder: he forwarded the sms to Ainy."

I think the crux is that you can't simultaneously say something is a blunder while also saying that it is a blunder because someone did it. To understand the original, you would need to know already that sending the sms to Ainy was a blunder.

Consider:
1. What Mac did was good, since he threw the ball over the fence. (Confusing; the second clause does not expain why what Mac did was good, but the 'since' implies that it does.)
2. Mac did a good thing by throwing the ball over the fence.
Only 2. asserts/explains/claims that throwing the ball over the fence was good.

[zaffar]

Thanks a lot sir. Your explaination really helped me out.