The following diagram is a quick description of what an email can go through between your software and the final destination of the email:

The above is really used to illustrate all the hoops an email that originates in the software gets to it's final destination. By following the suggestions we have included below you will improve your emails chances of getting through all those hoops. An email must pass through numerous tests and comparisons to reach an email account and it's up to you to find a good configuration within your Geo admin tool so your emails do not fail those tests.