Missing “some” emails on Outlook and devices.

I have a Verizon email account and have it set up on my computer using outlook, as well as on my iPhone, iPad, etc.  For some reason "some" incoming emails are not being delivered via outlook or on my other devices.  However, I can access them on the Verizon site.  I do not use the Verizon site to manage email, as I prefer outlook or my devices.  I have not found any rhyme or reason as to why this is happening.  Another words, I am not seeing where they are generated from the same type of email, be it gmail, comcast, yahoo, etc.  Any suggestions?!?