Chris Walker's Blog

Unfortunately, there is not a whole lot that we can do to prevent your emails from ending up in other email providers spam system. A number of these email providers use different spam firewalls, which are similar to the spamassassin service I use here. Each of these spam firewalls provide different types of filtering, different methods, and their configurations can vary from provider to provider drastically.

Follow up:

Most spam filter service follow an algorithm, and a list of 'bad words' that are generally seen in spam to determine what is potentially spam. Each message that comes in is checked by this for almost 100 different things that it thinks is spam. Some spam filters like 'barracuda' a common spam filter/firewall for small to mid-sized companies listen to a central server which delegates all of this, but can be controlled from the providers side.

Long story short, there are a lot of variables to consider to find out why it is being marked as spam and there is not a lot that your domain provider can do, however, I do have some possible suggestions that you can try below.

First, and this is always recommended. Ask your customers or vendors to whitelist *@ [something like *@devnetonline.net] in their email client and spam filter. This tells your vendors mail server to not even bother to
check your emails for spam, and pass them through. This fixes the problem 95% of the time.

Some of these spam filter services even rank email based on 'length' and think that short emails could be spam. Adding more information into the email, such as a 'signature' of your name, email address, phone number, or similar might help.

Some spam filter services check for words that raise flags that it could be spam, things like 'stocks' 'buy' 'cash', basically think about all the spam you've seen, you can see major patterns in the words that are contained within the spam messages. Rewording, or even decreasing the frequency of certain words may help, this can be a bit hard depending on the types of messages you send.

Last, the application [outlook, thunderbird, eudora, mac mail, etc] that you use to send email can sometimes even be a potential problem. Each email message that is sent contains a specific set of 'headers' that are used to identify the sender, recipient, originating domain, and more for the email. Sometimes, with older email clients, these headers are not updated to match everything that they should, and sometimes even send the bare minimum mail headers, leaving the rest up to the mail servers, spam filters take this very badly and will mark a message as spam. Upgrading your email client will generally fix the problem here.