What is an e-mail address?

An e-mail address is the address by which both the sender and the recipient of an e-mail message are clearly identified worldwide.

An e-mail address consists of two parts separated by an @ sign:

• The local part is before the @ sign.
• The domain part is after the @ sign and contains the domain name of the owner of the address.

Usually the whole email address is written in lowercase letters, because there are some older mail servers that distinguish upper and lowercase letters and this would lead to confusion.

Guidelines for the local part of the e-mail address

The local part must be a string that is unique with respect to the domain part. According to RFC 5322, this string may only contain letters and digits and certain other characters: A-Za-z0-9.!#$%&'*+-/=?^_{|}~.

The entire local part (or a section of the local part surrounded by dots) may be enclosed in double quotation marks (e.g. “MaxMustermann”@example.com or Max. “Musterboy”.Mustermann@example.com).

Within these quotation marks - in addition to the characters already mentioned - spaces and the characters “(),:;<>@[] (according to ASCII (decimal): 32, 34, 40, 41, 44, 58, 59, 60, 62, 64, 91-93) may be used. (e.g. “Max Mustermann”@example.com.) However, the characters \ (backslash) and " (double superscript) would have to be “masked” in it by means of a backslash character.

In addition, comments can be inserted inside round brackets. However, this is only allowed at the beginning and at the end of the local part. (Example: MaxMuster(comment)@example.com or (comment)MaxMuster@example.com).

All characters above ASCII code 127, including umlauts, are generally forbidden.

There must not be a period at the beginning and end of the string.

See also:
Wikipedia: E-Mail Address