Your Green Party inbox should appear in the app. Emails will start syncing - this may take a minute or two depending on how many messages you have. Send a test email to yourself to confirm both sending and receiving work.
"Authentication failed" error: The most common cause is a mistyped password or username. Make sure the username is your full email address (e.g. officer@localparty.greenparty.org.uk), not a shortened version. The password is case-sensitive.
Gmail app says "Couldn't connect to server": Double-check the server addresses and ports match the quick reference table above. Also make sure you selected Other at step 4, not Google or Outlook.
Can receive but cannot send: Confirm the outgoing server settings are smtp.stackmail.com, port 465, with SSL/TLS. In the Gmail app, go to the account settings and check the outgoing server configuration.
Emails are slow to arrive: In the Gmail app, IMAP accounts are checked on a schedule rather than pushed in real time. Go to your account settings and set the sync frequency to every 15 minutes for the best balance of timeliness and battery life.
Samsung Email keeps asking for the password: Make sure battery optimisation is not killing the app in the background. Go to Settings > Battery > Background usage limits and make sure Samsung Email is not on the sleeping or deep sleeping apps list.
If none of these solve it,
raise a ticket and include your device model, Android version, and which app you are using.