Can I retrieve the truncated part of a truncated email which
contains the following "message source" line?

iPlanet-SMTP-Warning: Lines longer than SMTP allows found and truncated.

Server (The SMTP server) keeps log of all emails that has been receive
and sent, only the admin of the server can access them.

Otherwise, you as a client, you cannot see the truncated part since
all you have is the bit of the email, the other truncated part is not
attached to your email.