After an upgrade from a 5.3.X version to 5.4, it could happen that the SSL checkbox, in mailing settings (General settings / Bounce settings), is greyed out.
Or you receiving many error in the CCA log file like this:
2017-12-12 10:35:36.572 Thread 9128 ERROR POP3 Exception while checking server pop.xxx.com:995 [SSL] (username: firstname.lastname@example.org) for bounced emails. Error message: Failed to create SSL socket
This is due to the fact that the SSL DLL's are still built with Visual Studio 2013. And 2013 is not installed by default during an upgrade.
So, to sort this out, you need to install the vcredist_x64 (vc2013b).exe that you can find, in standard installation, on CCA server, in D:\Askia\AskiaField\Packages\VcRedist.