In more detail
The local part can be up to 64 characters and the whole address up to 254. To deliver a message, the sending server looks up the domain's MX records to find the receiving server; only that server interprets the local part. The domain is case-insensitive, and although the local part may technically be case-sensitive, almost every provider ignores case. Many providers support subaddressing with +, so [email protected] is delivered to [email protected].
Why it matters
Overly strict form validation rejects valid addresses containing +, an apostrophe (o'[email protected]) or a newer TLD. A format check catches typos such as a missing @ but cannot prove the mailbox exists; only sending a message and having the owner confirm it does. Whether mail is trusted depends on the domain's SPF, DKIM and DMARC records, not on the address itself.
Example
[email protected] has the local part first.last+news (15 characters) and the domain mail.example.com. A quoted local part such as "john doe"@example.com is valid under RFC 5322, but many forms and some mail systems refuse it.