What are some alternatives?
When comparing Altospam and SpamSieve, you can also consider the following products
Scrollout F1 - Scrollout is a free and easy to use anti-spam email gateway designed for Linux & Windows administrators without advanced email security experience.
Mailwasher - Over 8 million people use MailWasher as their spam filter
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
CryptoPrevent - CryptoPrevent is an Anti-Virus/Security Software Supplement, originally designed in late 2013.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.