If emails are not being retrieved, one reason could be that an error is being thrown.  You can turn on the WordPress error logging flags and check the debug.log file to see if a related message is logged in there.

If the error message in the log file includes “Malformed Header” as part of the message you should check to see if there your SPAM filters are adding text to your email messages.

SPAM ASSASSIN, for example, adds some text to the headers of each email message but does it in a non-standard way.  The easy fix is to turn off spam checking for your support inbox or, if available, disable the option to add spam information headers to incoming email.

Here is an explanation on StackOverflow.com that discusses why SPAM ASSASSIN might cause an issue: https://stackoverflow.com/questions/48243201/zend-mail-headers-issue-malformed-and-content-preview

Another Example

Here is another example where a SPAM filter added a header to an email that does not conform to the EMAIL RFC standards.


Why This Example Is An Issue

In this particular case the spam filter software added data to the email headers that included a sequence of line breaks – a violation of the RFC standards.  Something like this will cause Awesome Support to reject the email message.