Go directly to your e-mail account and click on "Add email account". Now select "Google" if it is an account there, for all other providers please select "Other".
Enter your username and password in the following dialog. All other settings should be recognized automatically. If everything worked out, you will now see your external account under your folders on the left.
Some providers need some additional adjustments in their preferences
We will demonstrate this with the example Google Gmail
1. Please log into your Gmail account
2. Choose Settings by clicking on the small gear and then on Settings
3.Please choose Forwarding and POP/IMAP
4. Please enable IMAP by clicking on Enable IMAP under Status: IMAP is disabled and save these settings
5. Now please log into your Google account (not your Gmail account!) and click on Security
6. Scroll down and under Less secure app access please click on Turn on access
7. A new window will appear. Under Less secure app access click on the switch to activate less secure apps.
8. Now it reads Allow less secure apps: ON
Congratulations! You have now set up Gmail as an external account in your mailbox.org-account.
Please note that accounts included in this way will not be synchronized if you set up your mailbox.org mailbox on external devices.