Hello!

You would be better off by restricting users ability to initiate SMTP and POP3/IMAP protocols outside the company internal network. Leave the ports associated with email traffic open in firewall towards internet for your corporate mail server only.

Note that it still leaves open the possibility to create in Pmail another identity (of a coworker for example) and send emails via coproprate mail server by pretending to be somebody else. So You should also look how to close down this possibility in your corporate smtp server as well. 

After setting up the company e-mail account, I would like to lock down Pegasus' configuration so that users will not be able to create new accounts to connect to other e-mail servers like Gmail, Yahoo, etc. Is this possible to implement in Pegasus? Thank you

Interesting question.  New POP3 and SMTP accounts are stored as .PND files in the users mailbox directory.  IMAP definitions are stored in a file named IMAP.PM in the same location.  The user needs full permissions to this directory in order for Pegasus Mail to function properly so my initial answer is that you can not block the creation of these accounts.  Thinking further though, I wonder if it is possible to block creation of these specific files with a Group Policy (this gets beyond my knowledge level).  If you attempt this you should not block creation of all .PM files because there are many that Pegasus Mail depends on.  The .PND files are a different story, they are exclusive to POP3 and SMTP accounts.  I do not know how Pegasus Mail would react to the blocking of such files.