On webmail, you can compose or reply an email using a group mail, or your colleague's email address. To do so, you need to first set up these email addresses as your alternate identities.
1. On your webmail, click into General. 2. On “Alternate Identity”, enter the full name, email address and reply-to address on the given rows. 3. Click [Update] to save the records.
Note that if you add an email address that is out of your domain, there is a high chance that the emails you sent using this email address will be blocked by the recipient's server, due to different sender and original email addresses used, which violates the standard SPF checking.