Creating a subject line is no simple matter. It can literally determine the success or failure of your entire email campaign. There are some general rules to follow and it isn't so much what "to do" it's more what "NOT to do". Following these simple guidelines when doing an email blast will make the spam filters and you happy.
The simplest rule is don't use exclamation marks. Spam filters will love to pop you for this one. Another easy one is no caps. This is pretty self explanatory. All caps = All spam.
Never use the, oh so tempting, "free" or "huge savings". This is always the most controversial of the rules since adding words like this can double or even triple your open rate. The best bet is to try it at your own discretion and carefully monitor your results after every change.
You even want to avoid any type of currency symbol like the dollar($) or pound(£). This is very similar to the 'free' concept because whenever using a currency symbol it's usually involved with 'you won $10,000!' This will surely get you to click this headline or better yet 'spamline' and the spam filters know this.
The same thing goes with branded words like 'android' or 'apple' since money isn't the only thing people love these days. Everyone would love to win a fancy new smart phone by clicking an email. Expect to join the spam folder with the rest of the mass email trash.