Successful subject lines are striking and tempting, but some are also boring ,desperate and what's worse, deceivers. The idea of writing a successful subject line is to get the client to open your email, but there are good ways and bad ways to achieve this.
Capital Letters: Sometimes when you want to emphasize important words they are spelled all in uppercase, but this gives the feeling that you are screaming and it’s the last thing you want to convey to a potential customer. So we recommend to just add an exclamation mark or write in capital letters the beginning of each word in your subject line, for example: Last Day of Sale.
Abusing Special Characters: Similar to using uppercase letters, you may seem to be screaming if you use more than one punctuation mark at a time. You can achieve your intention of highlighting its importance by using only one punctuation mark.
Symbols: Symbols such as smiley faces and hearts are fun, but using them often becomes annoying. If you are using these symbols make sure they are appropriate for your content and audience, and only use one at a time and occasionally.
False Alarm - URGENT: If an email is not really urgent do not add the text urgent (or similar words) in your subject line since this is misleading and may cause your client to lose interest in your future campaigns.
Repetition: If you are sending a series of emails, lets say a promotion in the course of three weeks, do not use the same subject over and over again. This may cause the customer to ignore your email or even worse delete it.