ezcMailParser not able to retrieve bcc from a mail
In my experience, reading the Bcc field is not a reliable source of information, because there is not always available data. Thanks to your question I checked the RFC in response to your question and learned why!

What's happening

See https://tools.ietf.org/html/rfc2822#page-22


  The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains    addresses of recipients of the message whose addresses are not to be    revealed to other recipients of the message.  There are three ways in    which the "Bcc:" field is used.
  
  In the first case, when a message    containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is    removed even though all of the recipients (including those specified    in the "Bcc:" field) are sent a copy of the message.


When sender and recipient are on different servers, this is the behaviour you'll observe. (Outlook.com => Gmail.com result)


  In the second    case, recipients specified in the "To:" and "Cc:" lines each are sent    a copy of the message with the "Bcc:" line removed as above, but the    recipients on the "Bcc:" line get a separate copy of the message    containing a "Bcc:" line.
  
  (When there are multiple recipient    addresses in the "Bcc:" field, some implementations actually send a    separate copy of the message to each recipient with a "Bcc:"    containing only the address of that particular recipient.)


When sender and recipient are on Outlook.com, this is the behaviour you observe. (Outlook.com => Outlook.com result)


  Finally,    since a "Bcc:" field may contain no addresses, a "Bcc:" field can be    sent without any addresses indicating to the recipients that blind    copies were sent to someone. 


So - it's not a difference in Outlook vs GMail behaviour, so much as a difference in how MTAs handle internal and external deliveries. I still would not expect to see other Bcc recipients appear though.

See also this similar question/answer.

How to handle this

My take is: if you're parsing an email and the receiving address is not in To or Cc, it’s a fair bet that it was either Bcc'd or the receiving address is the final recipient of an email alias.

Beyond that, don’t expect to know who’s in the Bcc field - that’s what it’s for.