Do images always negatively affect email deliverability?

Images themselves don't directly hurt deliverability, but how they are used does. Large file sizes, a high image-to-text ratio (especially image-only emails), or embedding them directly (instead of linking to hosted images) can all negatively impact whether your email lands in the inbox. Spam filters look for a balance of text and images.

Can PDF attachments cause emails to go to spam?

Yes, PDF attachments can affect deliverability, primarily due to file size, potential security concerns (like malware), and how spam filters scan attachments. Transactional emails (e.g., invoices) are often accepted with small PDFs, but for marketing emails, linking to an externally hosted PDF is generally safer and recommended.

What are the best practices for using images and PDFs to maintain good deliverability?

For images, optimize file sizes, maintain a balanced image-to-text ratio, and host images externally (linking to them in the email). For PDFs, avoid direct attachments for marketing emails, instead linking to PDFs hosted on your website or cloud storage. If an attachment is necessary (e.g., transactional), ensure it's very small and properly named.

Why are image-only emails considered bad for deliverability?

Image-only emails are a significant red flag for spam filters because spammers often use them to bypass text-based content analysis. Such emails lack sufficient text context, making them appear suspicious to inbox providers and increasing the likelihood of them being marked as spam or blocked (or blacklisted).

Do images in email and PDF attachments affect email deliverability? - Content - Email deliverability - Knowledge base

When crafting emails, a common question I hear is whether including images or PDF attachments will hurt deliverability. It's a valid concern, as the goal is always to land in the inbox, not the spam folder. The short answer is: it's complicated, and depends heavily on how these elements are implemented.

While images and PDFs can significantly enhance your message, they also introduce factors that spam filters scrutinize. Understanding these nuances is crucial for maintaining a strong sender reputation and ensuring your emails reach their intended recipients.

The impact of images on email deliverability

Images in emails are powerful tools for engagement and branding. They can make your content more appealing and easier to digest. However, their impact on deliverability isn't straightforward. Images themselves don't inherently send your email to spam, but certain practices surrounding their use can trigger filters and affect whether your message gets through.

One of the primary concerns is the total email size. Large image files can significantly increase the overall size of your email. Many email providers have size limits, and exceeding these can lead to messages being rejected or bounced, never reaching the recipient's inbox. Even if the email isn't outright rejected, a heavy email might take longer to load, leading to a poor user experience and potential disengagement.

Another factor is the image-to-text ratio. Emails that are heavily image-based, or even image-only, can be a red flag for spam filters. This is because spammers often use images to bypass text-based filters, embedding malicious links or deceptive content within an image. A healthy balance between text and images is generally preferred to signal that your email is legitimate and contains valuable information.

Bad image practices

Image-only emails: These are a major red flag for spam filters, as they mimic common spamming tactics. Inbox providers might view these with suspicion, leading to lower deliverability rates.
Excessive image size: Large files increase email loading times and can trigger size limits set by mailbox providers. This can result in your emails being bounced or delivered to the spam folder.
Poor image-to-text ratio: If images dominate the email content, spam filters might interpret this as an attempt to evade content analysis. Maintaining a balanced image to text ratio is important.

Good image practices

Balanced content: Combine images with sufficient text to create a rich and informative message. This signals legitimacy to spam filters.
Optimized image size: Compress images to reduce file size without compromising quality. Tools like TinyPNG can help.
Hosting images externally: Link to images hosted on a reliable server instead of embedding them directly, reducing email size.

PDF attachments and deliverability challenges

PDF attachments, while seemingly innocuous, can present significant deliverability hurdles. While many reputable businesses send transactional emails with PDFs, such as invoices or receipts, attaching them to marketing or bulk emails generally carries a higher risk. Mailbox providers are wary of attachments due to their potential for carrying malware or spam, and PDFs are no exception.

Similar to images, the file size of PDF attachments is a critical factor. Large PDFs can push your email over common size limits, leading to delivery failures. Furthermore, spam filters often scrutinize attachments, and complex or unusually formatted PDFs, or those with suspicious naming conventions (e.g., including domain names), can trigger red flags, causing your email to be blocklisted (or blacklisted).

The type of content within the PDF also matters. If a PDF is essentially just one large image, it can face the same issues as an image-only email. Some filters even specifically look for malware hidden within longer PDFs. Therefore, while PDF attachments are common for certain types of email communication, their use demands careful consideration and adherence to best practices to avoid deliverability issues.

Key considerations for PDF attachments

File size limits: Most email providers have strict limits, typically around 10-25MB. Exceeding these limits leads to bounces, so it's vital to keep email file sizes in check.
Security risks: PDFs can carry malware, making them a target for heightened security scrutiny by inbox providers and email blacklists
Content type: If the PDF is merely an image, it inherits the deliverability challenges of image-heavy emails.
File names: Certain file names, especially those mimicking domains, can be hard bounced by providers like Gmail.

Best practices for images in emails

To ensure your images enhance, rather than hinder, your email deliverability, several best practices should be adopted. First and foremost, optimize your images for web use. This means compressing them to the smallest possible file size without sacrificing visual quality. There are many online tools available that can help with this, ensuring your emails load quickly and don't hit size limits.

Second, always aim for a balanced image-to-text ratio. While a hard rule doesn't exist, emails predominantly composed of text with supportive images tend to perform better in terms of deliverability. This also improves accessibility for recipients who may have images turned off by default in their email client. Always use alt text for images, as this provides context even if the image doesn't load and is used by screen readers.

Third, consider how you host your images. Embedding images directly (CID embedding) increases the email's file size. A better approach for marketing emails is to host images on a reliable server or Content Delivery Network (CDN) and link to them in your email's HTML. This way, the images are downloaded by the recipient's email client only when the email is opened, keeping the initial email size small. This strategy can significantly improve deliverability, especially for Gmail deliverability.

Finally, avoid using images as a substitute for important text, especially in critical elements like calls to action or key information. Some email clients block images by default, and if your message relies solely on images, recipients might miss vital details. Always include a plain-text version of your email and ensure that essential information is present in both.

Optimizing image use for deliverability

Compression: Reduce image file sizes using compression tools. Aim for optimal balance between quality and file size.
Alt text: Always include descriptive alt text for accessibility and to convey meaning if images don't load.
Image-to-text balance: Ensure your email has a healthy ratio of text to images, avoiding image-only designs.
External hosting: Host images on a CDN or external server and link to them within your email code.

Strategies for handling PDF content

While attaching PDFs can be problematic for email deliverability, there are effective strategies to share documents without triggering spam filters. For marketing materials like brochures or ebooks, the best approach is to host the PDF on your website or a cloud storage service (e.g., Dropbox) and include a direct link to the document in your email. This keeps your email lightweight and avoids attachment-related scrutiny. Several sources, like Mailjet, also recommend this approach.

If a PDF attachment is absolutely necessary, such as for transactional emails like invoices or tickets, ensure the file size is minimal. Optimize the PDF using compression tools before attaching it. Also, be mindful of the PDF's content and its filename. Avoid filenames that might appear suspicious or contain unusual characters.

For instances where multiple large PDFs need to be sent, consider zipping them into a single archive, or even better, provide links to each document individually. This minimizes the risk of hitting attachment limits and reduces the chance of triggering spam filters. Always test your emails, especially those with attachments, to see how they perform across different email clients before sending them to a large audience. Understanding PDF linking impacts can prevent deliverability problems.

Method	Impact on Deliverability	Best use case
Direct attachment	Higher risk due to file size, security scrutiny, and potential for blocklisting. Can lead to emails being sent to spam.	Transactional emails like invoices or receipts, where direct attachment is expected and often small.
Linked PDF (hosted externally)	Significantly lower risk. Email size remains small, and content isn't directly attached. Focus shifts to link reputation.	Marketing materials, e-books, reports, or any content that doesn't require direct attachment for administrative purposes.

Views from the trenches

Best practices

Optimize all images for web, compressing them to reduce file size without quality loss.

Maintain a healthy image-to-text ratio, ensuring sufficient text content alongside visuals.

Host images on a reliable server or CDN and link them in your email, rather than embedding them directly.

Common pitfalls

Sending image-only emails or emails with an extremely high image-to-text ratio, which can trigger spam filters.

Attaching excessively large image files or multiple large PDF documents, leading to bounces or delivery to spam.

Embedding images directly into emails, increasing overall email size and potential deliverability issues.

Expert tips

Consider transactional emails as a special case for PDF attachments, as they are often expected by recipients.

Regularly monitor your email deliverability and sender reputation to catch potential issues early.

If using an email testing tool, interpret 'no image' warnings in context with your overall email strategy and sender reputation.

Marketer view

Marketer from Email Geeks says there is no general need for an image in an email for better deliverability, despite some tools suggesting otherwise.

2024-09-30 - Email Geeks

Expert view

Expert from Email Geeks says emails with multiple links and no images are more susceptible to being marked as spam, especially when combined with domain or IP reputation issues, and if the message is sent multipart.

2024-09-30 - Email Geeks

Navigating images and PDFs for optimal deliverability

Ultimately, images and PDF attachments can affect email deliverability, but not in an absolute sense. It's not about avoiding them entirely, but rather about using them strategically and responsibly. By optimizing image sizes, balancing text with visuals, and opting for linked PDFs over direct attachments where appropriate, you can leverage their benefits without incurring deliverability penalties.