It is hard to set a firm limit for email size, firstly because this is very hard to measure in any editor, and secondly because deliverability is strongly influenced by reputation. However, we would suggest that 102KB is a threshold above which deliverability rates will begin to decline.

The easiest way to judge the size of your email is simply to look at the preview: longer emails with many images are larger than shorter, text-only ones. Attachments, which should always be avoided in marketing emails, will also pile on the bytes.

Aside from these, there are many hidden elements that will affect the size of an email.

• Images are the most obvious culprits. Reduce the size of inline or background images and remove unnecessary ones. Most importantly, host your images externally.
• Responsive design is an essential part of today’s email marketing but this also carries hidden weight, so factor this into your estimation.
• Text versions are critical to ensure good deliverability and although small in size, it all adds up.
• Overly complex HTML can increase the size of an email with no added benefit. Make sure that you remove any redundant code.
• Hidden formatting tags from content editors. If you write your content in, for example, MS Word, and copy & paste it into your editor, it can happen that hidden tags are copied across. Not only will these increase the email size, but they can also cause the text to render badly in some mail clients. If you use such an external editor, we recommend to clean up the content by pasting it first into a plain text editor such as Notepad.
• Line breaks and spaces are also useful to pay attention to. Eliminating the unnecessary ones reduces the message size.

A multipart email can still fall foul of spam filters if:

• It does not contain all the required headers and MIME parts (see below).
• The message has only the text or HTML MIME parts instead of both.
• The message has only text and no images.
• The text version is very different from the HTML version.
• Too many images and not enough readable text is sent, so the overall HTML has a low text-to-image ratio. The ideal ratio of text to image area for the HTML is 60/40. Making full use of the Text version of the email is a good way of boosting this ratio.

Below is a more technical breakdown of what constitutes a multipart message.

The most important role that a text version plays in a multi-part email relates to deliverability. However, they are also a good back-up for devices which can only render text, and you should always consider that some people may simply prefer to receive your messages in this format.

Text versions improve deliverability for the simple fact that spammers rarely invest the time to create one. Therefore most spam filters focus on this and the text-to-HTML ratio of an email when assigning it a spam score.

To calculate the text-to-HTML ratio, the spam filters decode the entire message and count the characters in plain text versus those inside HTML tags. Since images, formatting and links take up a lot of HTML code, having a text version is a great way of boosting your plain text count and getting above the threshold below which a spam filter will treat your mails as suspicious.

You can take 30:70 as a benchmark threshold ratio for text to HTML.

When creating your text version, keep it short and simple. Take the most important information from your email and present it in a clear visual way.

Also remember that you will not have visual aids such as colour or font size so pay attention to the visual effect of the text version and make use of white space. Make it easy for the reader to scan the message and don’t use dense blocks of text that will put them off.

Here are some useful tips to keep in mind when creating your text version:

• Structure the content clearly so that reader knows at once what you are trying to tell them.
• Clearly identify your call to action, for example by using angle brackets as arrows: >>> Product now available! <<<.
• Use simple characters such as *** and +++ to break up paragraphs, or to identify headings.
• Avoid less common ASCII characters such as copyright and TM since many are not supported by all mail clients.
• Include a link to the online version.

There are also some pitfalls to avoid:

• Take care when using ALLCAPS as many spam filters do not like them.
• Make sure that the content is similar to the HTML version – don’t just add text to up the character count. In particular, if you include any links (see below) the links and especially the domains should be the same as in the HTML version.
• Don’t be tempted to leave the header and footer out of the text version. This is still a marketing email and you will need to include your company information, unsubscribe link, etc.
• Don’t be lazy and use the same standard text version for all emails – spam filters will pick up on this.

To sum up, if you can take the time to create a quality text version, do so. If you cannot, then at least take advantage of the simple functionality offered by Emarsys for auto-generating it (see below). This might not look particularly attractive to readers, but they will at least help your deliverability.

You need to pay particular attention to links in text versions, since you cannot hide them behind an image or a link name. HTML emails traditionally include a large number of links and these can quickly obscure your primary message and make the text version unreadable.

Aside from the standard links that every email should include (e.g. online version, unsubscribe, contact, etc.), make sure your the links for your primary calls to action really stand out. Text versions are often read on mobile devices and it is unlikely that the reader will make more than one click, so make it count.

In particular, you should carefully consider which links to track, as these make the link syntax even more visually unappealing. Consider these three examples of a link:

• HTML:
  
• Text version, untracked:
  
• Text version, tracked:
  

In our opinion, it is worth losing some tracking to ensure a better experience for the reader. If you do want to track links, use a shortened version, but avoid publicly-used link shorteners as they can affect your spam score.
And finally, remember to show the link as well as the link text. Click here is useless without a link, and a link without a clear explanation is likely to be ignored.