Its also recommended to send a test message to a spam-protected email account and/or a webmail (e.g. freemail) account to verify the spam rating of the message.

Send out your newsletter messages to subscribers only and allow them to unsubscribe!

Common issues:

The header often contains details about the spam-rating:

e.g.

1.1 MPART_ALT_DIFF BODY: HTML and text parts are different
Include a plain-text part to your HTML message. (use the "Extract ..." function)

1.2 TVD_FW_GRAPHIC_NAME_MID BODY: TVD_FW_GRAPHIC_NAME_MID
Rename the inline image e.g. use 1.gif instead of header.gif

1.4 SARE_GIF_ATTACH FULL: Email has a inline gif
Use a different format e.g. PNG instead of the GIF image.

0.8 HTML_TITLE_SUBJ_DIFF HTML_TITLE_SUBJ_DIFF
Replace the title tag in the html sourcecode. (remove <title> ... </title>)

1.3 MISSING_SUBJECT Missing Subject: header
Add a subject to your message

1.7 SARE_GIF_STOX Inline Gif with little HTML
Add more text/html content or replace/remove image.

0.6 HTML_IMAGE_RATIO_02 BODY: HTML has a low ratio of text to image area
2.5 HTML_IMAGE_ONLY_16 BODY: HTML: images with 1200-1600 bytes of words
Add more text/html content or remove image.

1.6 MY_CID_AND_CLOSING SARE cid and closing 
1.5 MY_CID_AND_STYLE SARE cid and style 
1.6 MY_CID_ARIAL2_CLOSING SARE cid arial2 closing 
1.6 MY_CID_ARIAL_STYLE SARE cid arial2 style 
1.5 MY_CID_AND_ARIAL2 SARE CID and Arial2
Remove style/font tags (<div>,</div>,<font>,</font>) before and after the <img ...> tag. If its not possible try to change the font e.g. use MS Sans Serif instead of Arial.