If you cannot login after multiple tries. There could be a few causes.
1) Few email providers disable IMAP access by default. You need to explicitly enable it. For instance, for GMX, this is how you do it.
2) The required port might not be open. In some firewalls, ports like 143 and 993 are closed. Try it on another network and see if it works. You probably need to fiddle with the router to get it working.
3) If you have a network monitor like LittleSnitch installed, it can be blocking the IMAP access. You need enable it in their admin panel.
4) If you have 2 factor authentication enabled, you need to generate app specific password to get it working.
You can find our other guides here:
Office 365 Troubleshooting Guide
If you need help with setting up your account, please feel free to contact us here. :)