By default, when you send email from Salesforce from Send an Email button, you will not select the email sender, it will be user name with corresponding email address.
But, in some case, user need to send email using other email address. Example: for team service.
Here few ways configuration you can do out-of-the-box from Salesforce:
1. Outgoing Email Settings
This setting do not need admin to enable or change it, user can do it by themselves. This setting is good when only effect to a few user and do not need to change the sender user name.
Go to Setup | My Stettings | Email | My Email Settings
Look for information on Email Address and change the email address.
You need to verify the email address, Salesforce will send email address to the new email address for you to verify. Once done, you will see option From with email picklist when send email, but username will remain the same. This email will be default email address when send email.
2. Organization-Wide Email Addresses
This setting will need system administrator involvement and will effect to users in a profile, many profiles or ALL users.
Go to Setup | Email Administration | Organization-Wide Addresses
Click Add button and enter Display Name, Email Address and Select Profile or All Profiles.
When you send email, this email will be available as a picklist for user with profiles allowed to use this Organization-Wide Email Addresses. You'll notice the sender name is different with user name.