I am going to "try" to be as short as possible.
1. Check your emails (especially one-to-one out of your CRM) by making dummy email accounts on major email providers (aol, yahoo, gmail, etc.) and this should be repeated monthly (and always make a new inbox). This should be done on all templates - We have noticed large ebrochure emails that are heavy with images and links will always be filtered into SPAM/JUNK/PROMOTIONS.
2. Get yourself a dedicated IP for large email campaigns. Most email marketing companies offer this as an add-on (Mailchimp/Mandrill, etc.). Make sure you monitor your IP reputation score and that it never falls below 98%.
3. Day/Time - This is a big part of your success. You don't want to be sending emails when everyone else is doing the same; this will increase the percentage of deletes (without open), SPAM flagging, etc. Open up a dummy email account and sign up to companies that you believe your customers may get promotional emails from such as Priceline, Orbitz, Home Depot, Loews, WalMart, Macy's, Saks, Neiman Marcus, Fry's, Gap, etc. and make sure you time your emails to be sent during the lightest part of the day; this will increase engagement and open rate and will ultimately help with your reputation.
Here are some points from a company that we work with that has a lot of knowledge on email marketing campaigns:
1. Low Open Rate - If your emails are not being opened or are being deleted without opening this will affect your campaign.
2. Spam Complaints - Once users start marking your emails as SPAM then your emails will begin to automatically end up there or be blocked entirely. This is why you MUST have your own IP and outgoing email server so you can monitor the amount of emails being marked as SPAM, Bounced, etc.
3. Your Email List - Inbox's monitor your campaigns and if your emailing bad addresses this will hurt your reputation and will cause your emails to end up in places other than the INBOX. Make sure you are cleaning your email list from people that have not engaged in awhile or have inactive email accounts, bad emails, etc.
4. Physical Address/Unsubscribe Link - Make sure these are in all of your emails.
5. HTML vs TEXT - If you are using text emails then you have nothing to worry about (outside of a lower engagement rate). For those who use HTML formatted emails you better make sure your html is (1) code is simple & clean (2) max width of 600x800 pixels (3) mobile optimized (4) image to text ratio is good (don't send a bunch of images with no text) (5) Images are optimized for mobile (6) don't use crazy fonts
6. "Bad" Words - Don't use these words in your emails as they are flagged words considered to be possible spam.
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Here are some sites to check your IP and Email Rating:
http://www.emailreach.com/
http://isnotspam.com/