If you are on a departmental mailing list and have a need to send messages from that mailing list address in Gmail, you will want to set the address up as a "Send Mail As" account.
These instructions assume both accounts are on the swarthmore.edu domain. If not, contact the Help Desk for support.
In Gmail, open Settings from the gear menu.
Choose the Accounts tab
Click Add another email address you own. Enter a descriptive official name for the account and the email address. Keep Treat as an alias checked.
You will be asked to send a verification email. Click Send Verification.
The verification email will be sent to the list. Check your inbox for the email containing the confirmation code.
Enter the confirmation code on the confirmation window.
Now when composing an email, you will have the option to change the "From" address to the alternate email address.