An Ethereum address may contain a mixture of capital and lowercase letters that looks arbitrary. In a checksummed address, that pattern carries information: software can use it to catch certain transcription errors. It is useful, but it answers a narrower question than many people expect.
The ERC-55 specification describes a mixed-case encoding derived from the address itself. A compatible checker can compare the supplied letter case with the calculated pattern. This is an error-detection mechanism, not an identity certificate.
Correctly formatted can still mean the wrong recipient
A different address can have a perfectly valid checksum. Passing that check does not establish that the address belongs to the person, organisation or service you intended. It also does not prove that a website displaying it is authentic.
Think of the distinction as two separate questions: does this string satisfy a formatting check, and did it come from the intended recipient through a verified channel? A yes to the first does not supply an answer to the second.
Keep the original address intact
Use the recipient’s verified receiving instructions and the wallet’s supported address checks. Avoid retyping a long address or changing its capitalisation simply to make it look consistent. If software reports a mismatch, stop and confirm the original information rather than modifying characters until the warning disappears.
Do not infer validity just from seeing both uppercase and lowercase letters. The pattern needs to match the algorithm; visual variety alone is not a checksum. Some valid representations may also look different, so appearance is an unreliable substitute for validation.
The network and asset remain separate checks. This explanation concerns ERC-55 address encoding, not a universal rule for every blockchain or wallet. Before any transfer, follow the receiving service’s current instructions for the particular network and asset. A checksum is one useful guard against mistakes, with a clearly limited job.