Send the correct hostname with the HELO/EHLO command

Summary:
It sent the server's hostname previously, which some reject.

BUG: 387926

Test Plan: Ran nc as smtp server, uses the right hostname for EHLO now.

Reviewers: mlaurent, dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D9485