Nowadays most mail servers have built-in antispam filters; they are quite smart, but sometimes they fail and delete (or mark as spam and redirect to some “spam” folder) perfectly legal messages; this is a common problem with Hotmail and other very popular free mail services. If you feel a lot of your messages are filtered out and not delivered, here are some basic rules to avoid being named a spammer by mistake:

Always include plain text: “personal” e-mail messages, even if written using HTML format, always include a plain text part. If a plain text part is missing, most antispam filters become suspicious. When you finish composing your message, use  “Text from HTML” button to automatically create a plain text part from HTML.

Most of your message must be text; avoid including large images. Once again, antispam filters become suspicious when the image / text ratio is too high.
 

Don’t over-attach: attachments should only be used in small, personal lists; if you are publishing a newsletter, upload files on your website and use links instead (your subscribers will be happier too).

Avoid risky words that are commonly used by spammers and constantly flooding your mailbox.

Switch to Smtp: While Direct Send mode is very useful in many situations, unfortunately nowadays a lot of mail server refuse direct delivery in order to prevent spam. You should better use Smtp when you are mailing a large amount of recipients.