Definition
Every email address has two parts separated by an @ sign:
- The local part, before the @, identifies the mailbox. Example:
jane.doe. - The domain part, after the @, identifies where mail is delivered. Example:
company.com.
The formal rules come from RFC 5321 and RFC 5322. A few practical limits:
- The local part can be up to 64 octets.
- In practice the whole address should be no longer than 254 characters.
- Letters, digits and characters like
.,_,-and+are common in local parts. A dot cannot start or end the local part, and two dots cannot sit side by side unless the local part is quoted. - Domain labels are letters, digits and hyphens, up to 63 characters each.
The RFCs also allow some odd forms, such as quoted local parts with spaces and IP address literals. They are technically valid and almost never used for real mailboxes.
Why it matters
Syntax errors are the cheapest problems to catch and among the most common in collected data. Typical examples: a missing @, two @ signs, a trailing dot, a space in the middle, a comma instead of a dot, or a whole name pasted into the field.
The opposite mistake is just as real. Overly strict form validation rejects legitimate addresses, especially ones with a + tag, a long new top-level domain, or a hyphenated domain. Every rejected real address is a lost sign-up.
Syntax checks tell you an address could exist. They say nothing about whether it does.
How SimpleCold handles it
SimpleCold normalises each address and checks its syntax first. Failures come back Undeliverable with the status invalid_format and the reason "This isn't a valid email address." The check stops there, so no DNS lookup or mail server connection is spent on it.
Test any address with the free email syntax checker.