A Salesforce username must be globally unique across all Salesforce organizations, including production, sandbox, and trial orgs. It must be formatted like an email address, but it does not need to be a valid, working email. If an administrator attempts to create a new user with a username that already exists in any other Salesforce instance worldwide, the system will prevent the creation and display an error indicating that the username is already in use. This is the most common reason for being unable to add a new user with a specific username.

B. The username is restricted to a domain specific to my domain.

My Domain creates a custom login URL but does not restrict the domain of the username itself. Users can have usernames with any domain.

C. The email address used for the username has a contact record.

The existence of a contact record with a matching email does not prevent user creation. In fact, users can be created directly from contact records.

D. The email used for the username is not a corporate email address.

Salesforce does not validate whether a username's domain is "corporate." Any string in a valid email format can be used as a username.

1. Salesforce Help Documentation

"Add a Single User": Under the "Username" field description

it states

"A username must be in the format of an email address... The username must be unique across all Salesforce orgs

including trial orgs. The username is the user’s unique identifier for logging in to Salesforce."

2. Salesforce Help Documentation

"Guidelines for Adding Users": This document reinforces the uniqueness requirement: "Usernames must be unique across all Salesforce orgs. The username must be in the format of an email address (for example

[email protected])."

3. Salesforce Trailhead

"User Management" Module

"Add Users" Unit: This module explains the user creation process and highlights the key fields. In the section describing the username field

it emphasizes

"Every user must have a username that is unique across all Salesforce organizations."