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 (which can be a group mail, or your colleague's email address), the reply-to email address (when recipient reply to your email, this email address will be placed in the “To” field) into the available fields.

4. Click on the last field to select a signature, which will be appended at the bottom of an email when you use this identity for email compose. Find out how you can create signatures.
Skip this step if you do not want to add any signature.

5. You can add at most 6 alternate identities, with different email addresses and signatures.

6. Click Update to save the details.

IMPORTANT: 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 are used, which violates the standard SPF checking.

Now that you have created multiple identities, when composing or replying an email, you can click on the “From” dropdown box to select which identity/address you want to compose/reply an email.

The selected identity/email address will be shown as the sender at the recipient's received email.