(1) usual MTA configurations allow mail relaying TO any user with a local address, FROM any user of the hosts residing on local network/domain, and FROM authenticated users on the Internet;

(2) nevertheless, some MTA could require user authentication from hosts residing on local network/domain in order to allow relaying.

So, either you are trying to send outgoing mail connecting from home to your company MTA, or your MTA requires authentication even for internal hosts… Be aware, that any comparatively new MTA will refuse to authenticate users through unencrypted connection and using incompatible authentication method – thus the best way to proceed is to ask your sysadmin how to properly configure your mail client – type of encryption (SSL vs. STARTTLS), and type of authentication (lot of them around – plain, login, nlm, etc., etc.)