How can MTA identify a valid user?

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Because MTA is open source, it is possible for people to create their own customized versions.

Does the MTA client prevent fraud?

If the answer is yes, how?

Is there a positive use case? For example, a developer version of the mta client for personal use

[[TTH]BiZkiT]

the client does a crc check, and a developer version of the client i doubt we can get it.

I have a server running 24/7 with Mirc scripts you are welcome to check it out

[TTH]Clan Public https://servers.projectredivivus.com/

[SugarD-x]

MTA:SA is currently open-source. MTA 0.x is not yet. While I cannot speak on behalf of MTA:SA, [TTH]BiZkiT is correct about the CRC checks, (as well as other features), in the currently-released official versions of MTA 0.x. Development builds have lesser requirements due to testing purposes and building upon other newer security features, as well as removing outdated ones.

That being said, I suspect this topic was meant for the MTA:SA section of the forum based on the initial question's wording.