When you sign a consumer or business up to your email marketing list you want them to be a long-term reader - to achieve this you should establish some sense of consistency in your messages.
This does not mean that every email must always follow a strict layout and never drift from its shape or tone, but it does mean making a conscious choice about which areas you want to remain the same throughout, as this will tell you a lot about your priorities. In addition to this, you need to consider how consistency impacts the reader.
By including regular features you can tap into the human tendency for habit; for example, if you can get people interested in something like a 'top ten bestsellers of the week' list, then chances are they may like to see how the chart looks the following week, and the week after that, etc...
From your perspective as the firm putting together the email marketing campaign, the importance of consistency is largely down to its role in brand development, or at the very least the formation of a 'voice' within email interactions. You want clients to have a clear idea of what you represent and this can be difficult if all of your emails are completely different.
In contrast, if you keep the same tone, you are effectively reaffirming your identity with each message. The impact it can have on the reader is such that you should spend time crafting exactly how you want to present yourself. Perhaps you want a very structured layout throughout to maintain the air of professionalism? Or maybe you want to emphasise your creativity by including a newly-created graphic each time?
While consistency is good for a company to project their image, the real beauty is how it helps retain readers. On a fundamental level, if a person has enjoyed a piece of content, they will look forward to the next instalment if it is of the same ilk, so satisfy their demands and give them the sort of message they want.
The most obvious caveat to your quest for consistency is if your existing model is not working. Naturally, you need to make a decision about how long you will give a certain style of message before admitting it is not engaging with people. Try to continually gather feedback so you can make incremental improvements while holding on to the elements that are well received.
Another factor to consider is that sometimes it is good to shake things up if the status quo has been in place for a long time. Even if you are building up a great following with a certain format, you should eventually try to bring in changes to keep things fresh.
Think about how newspapers will have some columns running for years, while others only stay for a few months. You want to keep people's attention for as long as possible, which sometimes means dropping a feature and starting something new.