The match key
Everything depends on the key used to link your record to an external one. A corporate email domain is the most common and reasonably reliable. A registered company name is workable where the name is distinctive and unreliable where it is not. A personal email address on a free provider generally cannot be matched to a company at all. Choosing and standardising the key matters far more than choosing a vendor, because a weak key produces confident wrong answers.
The append and its precedence rules
Matched attributes are written back to the record. The decision that determines whether this helps or harms is precedence: what happens when the external source disagrees with what you already have. The sound default is that first-party data, anything the customer told you directly, always wins, and appended data only fills gaps. Storing where each field came from and when makes that rule enforceable rather than aspirational.
A worked example of match rate and accuracy (illustrative)
A team submits 1,000 lead records for enrichment. The source returns attributes for 620 of them, giving a match rate of 62%. That number alone says nothing about quality. To find out, they sample 50 of the enriched records at random and check each appended industry and headcount band against the company website and public filings. If 41 of the 50 are correct, sampled accuracy is 82%. Applied to the whole run, roughly 508 of the 620 enriched records carry correct values and around 112 carry wrong ones, which will be quietly acted upon unless confidence is recorded. The useful conclusion is not the percentage but the practice: a run with no accuracy sample has not been measured at all.