1. Regarding the part before the "@" symbol.

The period (dot) is repeated.

test..user@example.com
a...b@example.com
user....name@example.com

It starts with a "." (dot).

.test@example.com

The character immediately preceding the "@" symbol is a "." (dot).

test.@example.com
user.name.@example.com

Contains spaces (blanks) and full-width characters.

user name@example.com
ＵＳＥＲｎａｍｅ@example.com

It contains symbols that cannot be used.

Example: ( ), ; : < > @ [ ] ' "

2. Regarding the part after the "@" symbol (domain)

The period (dot) is repeated.

example..com

It starts with a "." (dot).

.example.com

It ends with a "." (dot).

example.com.

It starts with a hyphen (-).

-example.com

It ends with a hyphen (-).

example.com-

Includes symbols other than "." (dot) and "-" (hyphen).

exa_mple.com

Contains spaces (blanks) and full-width characters.

exa mple.com
ｅｘａｍｐｌｅ.com