When you send an e-mail from a shared mailbox, make sure you check the address the e-mail is being sent from. It may well default to sending the mail from your account, rather than the shared account.

To see the account it is being sent from click the ellipsis (…) and click Show From

send from 01
Click From to change who the e-mail is sent from send from 02
Click other email address… send from 03

Begin to type the e-mail address you want to send this e-mail from

Select the address you want to use from the suggested e-mails that will populate the drop down list

send from 04
Continue and send the e-mail as normal. send from 05