By design of the MTA that Protector uses (XMail) - for each recipient an email will be sent in a separate session.
while sending mail to internet or SMTP server from protector,One mail sent by sender where 3 recipient from yahooo and 2 from gmail , and it is creating each with in separate session 3 emails to Yahoo and 2 emails to Gmail.
So According to this specification, for example, if there are emails which have 10 MB of file attachment and 5 destination addresses are set, it consumes 50MB of bandwidth to communicate with the on-premises MTA. Regarding the mails which are large in size, it is a situation that consumes bandwidth of internet access line.
But expect to create single session, considering bandwidth of internet access line.