SMTP
Today, SMTP is used almost exclusively for mail transport, basically described in RFC 821. Unfortunately this RFC leaves (too) much room for interpretation, and the so-called "clarification" RFC 2821 doesn't clarify all issues.
Our Mail-Cluster
The most used mailserver (exactly: MTA, Message Transfer Agent) is sendmail. Unfortunately sendmail is well known for being slow, unreliable and has a very bad security track record. We use another solution: a cluster of qmail-ldap servers, a qmail variant fetching its user information from LDAP, running on a couple of OpenBSD/sparc64 machines, while the storage backend, LDAP servers etc are on OpenBSD/i386 and OpenBSD/amd64 machines.
Even nowadays email is often transferred unencrypted. Our mailservers support TLS, Transaction Layer Security (successor of SSL), for encrypted mail transfer to remote mail servers as long as they support TLS, too.
Aside from pop3 we are offering imap and a webmail interface for accessing your mail. SSL/TLS encryption is supported for all protocols. We do support SMTP AUTH for authentication within the SMTP transaction.
Mailaccounts, -addresses and much more can be configured using our Control Center.
By the way, the whole qmail-ldap documentation, Life with qmail-ldap, was written by our hostmaster Henning Brauer, as well as various patches incorporating new features. Of course hosted by us.
SPAM
These days, the amount of unsolicted eMail, usually referred to as spam, is so high that a meaningful use of eMail without countermeasures is impractical.
Thus, we are running a very sophisticated spam detection and prevention system. A plethora of techniques is being used, combined, from distributed blacklists to behavioral analysis of the connecting hosts trying to deliver eMail, to distinguish valid mail servers from spambots.
All these measures have one thing in common: we never just drop an eMail. When our system thinks a particular message is spam, it denies its acceptance, so that the connecting mailserver must inform the sender. We do signal a meaningful error message back for transparency.
This system, constantly finetuned by our staff, is very effective. Some customers use our mail cluster in front of their own mail systems just for the spam fighting part - while that is most efficient when our system has full control, it is still blocking most spam for them in this kind of setup.