A string shaped like an email address

The shape is three parts in a fixed order: a local part, one at sign, then a domain containing at least one dot.

The shape

Everything to the left of the at sign is the local part, and it belongs to whoever runs the domain on the right. Everything to the right is a domain name, which must contain at least one dot with a label on each side of it. There is exactly one at sign separating them in any ordinary address, and no spaces anywhere in the string.

The two halves follow different rules, which is the source of most confusion. The domain is case insensitive and drawn from a restricted set of characters. The local part is more permissive and is not guaranteed to be case insensitive, though in practice most systems fold it. Treat a difference in the domain as no difference at all, and a difference in the local part as a difference until you know otherwise.

Field order: local part, then @, then domain
At sign: exactly one, and it separates the two halves
Domain: at least one dot, with characters on both sides of it
Spaces: none in an unquoted address
Case: the domain folds; the local part is not guaranteed to
Trailing dot: a domain that ends in a dot is unusual and often a sentence full stop that came along

[email protected]
local part, one at sign, a domain with one dot

[email protected]
dots are ordinary in both halves; only the at sign divides them

Placeholders. Both are built from the word example so neither can be read as a real destination.

The at sign in three different places

The same character does three unrelated jobs across this family, and knowing which job it is doing is most of the skill. In an email shaped string, the at sign is a separator, and everything on both sides of it is meaningful. In a link, an at sign appearing before the first slash discards everything to its left and makes the host whatever follows, which is covered in full on the mirror link card. In an onion host it does not appear at all, because the label alphabet has no room for it.

That third case is worth stating on its own, since it comes up whenever somebody pastes a jumble of strings into a message. An onion address never contains an at sign. If you are looking at a string that ends in .onion and has an at sign somewhere in it, you are looking at two things that have been joined, or at a link whose host is not the part you were reading.

How to check it by eye

Starting from the at sign

Count the at signs. There should be one. Two or more means the string is either quoted in an unusual way or is not a single address.
Look right of the at sign for a dot with characters on both sides. A domain with no dot at all is not a public destination.
Look for spaces anywhere in the string. One space is enough to make it two strings that were printed next to each other.
If the address is wrapped in angle brackets with words in front of it, read only what is inside the brackets and ignore the words entirely.

What a wrong shape means

More than one at sign is the classic wrong shape, and there are two ordinary causes. Somebody pasted two addresses without a separator between them, or a client wrapped a line in the middle. The exotic cause, a quoted local part that legitimately contains an at sign, exists in the specification and is rare enough that meeting one in the wild usually means the string was assembled by hand.

The trap that matters here is not a wrong shape at all. Mail and chat clients display a friendly name in front of the actual address, and the friendly name is free text supplied by the sender. It can be a word, a company name, or another entire address. When it is another address, the display shows one destination and the string in the brackets is a different one, and nothing about that is malformed.

What is displayed

Example Support <[email protected]>
the words in front are a label, the brackets hold the address

What the label can hide

[email protected] <[email protected]>
the visible name is itself an address, and it is not the address

In the second line the text before the brackets is chosen by whoever sent the message, and the address in the brackets is the one that would actually be used. Only the bracketed part counts, and many clients hide it by default.

What a correct shape does not prove

Take a string with one at sign, a sensible local part and a domain carrying a dot. Here is the list of things that remain unsettled.

Whether the mailbox exists. The layout is a grammar check, and grammar has never established that anybody is home.
Who reads what arrives. A mailbox can be forwarded, shared by several people, or watched by somebody the sender never mentioned.
Whether the domain belongs to who you think. Domain names are registered by whoever registers them, and a familiar word inside one is just a word.
That the displayed name is related to the address. The two are independent fields, and only one of them is used for anything.
That a reply reaches the sender. The address a message is sent from and the address a reply goes to are separate settings, and neither is visible in the shape.

What this card is not

Nothing here concerns the contents of any message, which is a matter for the quoted message card, nor the signing of one, which starts at the keys hub. An at sign inside a link, where it changes which text is the host, is on the mirror link card rather than here. This card names no service, publishes no address, and takes no position on whether any particular address should be written to.