In many large organizations, some users will act as assistants to other users, managing their email, calendar, or both. In other cases, a user may occasionally send email as another user without requiring full access to their inbox. Other firms may want an anonymous in box like firstname.lastname@example.org to send mail from.
Delegating an entire inbox
Delegating an inbox allows a user (the delegatee) to access another user's (the delegator's) mailbox where they can send, receive, and manage the delegator's mail. To see how to delegate your mail, view the official Google documentation here.
Delegators will have access to almost all of Gmail's functions, with the exception of account-related settings and chat functions. This is great for executives who use an executive assistant to manage their mail and calendar for them. Delegatees can access the delegate's mail in the same method they would switch accounts; currently this means clicking your profile icon and selecting a different account.
Please note that when the delegatee sends mail as the delegator, the recipient will see "From: Delegator (sent by Delegatee)".
Using "Send Mail As" to send mail as another user
"Send mail as" allows you to send mail as a different user. See the official Google documentation for how to set it up. Note that the address you send email as and the address you'll receive replies to can be different. If you only set an address to send mail as, replies to that mail will go to your original email account. To get replies to go to the email you have send the mail as, you'll also need to set a "reply-to" address. Instructions are in the documentation linked above.
Please note that the recipient may still see an indication you send the email as someone else, depending on what email client they use. For example, they may see something like "from email@example.com on behalf of firstname.lastname@example.org". In Gmail, they should not see this extra information, they will only see the Send As address.
Automatic (Vacation) Responders
Please note that automatic responses, including vacation responses, use the SMTP Return-Path address, which is different from the Reply-To address and can not be (easily) modified.
Creating a Sender Mapping File
To more fully obfuscate a sending address, you can use a sender mapping file. To create one, enter the G Suite Admin Panel > Email Configuration > Advanced. There's many different options for this and it's dependent on your environment. Contact us for more information or carefully read the support article.
Using the first two methods listed here, it is still possible the recipient can see the user's original email address by looking at the full headers. Also, if support agents are trying to troubleshoot an issue, delegates will be asked to perform actions on the account. These actions should be performed by the delegate (not the delegatee - the real account holder) unless otherwise noted.
Google & G Suite cannot modify sending headers natively.