I have 6 e-mail accounts and I use Kmail, but I haven't seen any "Network tab" in its configuration dialog. Maybe it existed in one of the previous versions of Kmail? I currently use version 3.4.0.
In "Accounts" section, you can setup POP3/IMAP and SMTP servers and in "Identities" --- your email addresses and default SMTP server for each of the accounts.
Note that I use localized version of the KDE, so the names in doublequotes might be slighly different.