Email Deliverability Guide: How to Send Email Attachments Without Going to Spam

Email deliverability is not a simple delivery. This term stands for the successful placement of your messages into the recipient’s inbox, rather than getting them into spam or junk folders.

It matters for proper business communication and marketing. Without getting your messages into the inbox, you won’t get them seen. Apart from this, it also matters for your overall sender reputation.

Before you can improve email deliverability, you must know why your messages may not reach the inbox in the first place. For the most part, this depends on spam filters embedded in email systems. Hence, understanding the mechanism of their work and the factors that trigger them is a must.

What Is an Email Spam Filter, And How Does It Work?

An email spam filter is a set of algorithms. It’s meant to find unwanted or harmful content in emails. The ultimate goal is to prevent users from opening it.

There are several types of filters today:

• Content filters
• Blacklist filters
• Header filters
• Language filters
• Rule-based filters
• Bayesian filters

Each type of email spam filter targets a different thing. For example, language filters identify messages written in languages other than the user's, and Bayesian filters analyze which messages the user marks as spam to identify patterns and automatically get rid of similar emails. Regardless of the type, spam filters work in one of the following ways:

• By scanning the structure and content of an email.
• By analyzing the sender's reputation.
• By checking email authentication protocols (SPF, DKIM, DMARC).
• By analyzing email attachments and metadata.

Common Reasons Emails Go to Spam

Email deliverability can be hindered by:

• Suspicious or large (that exceed the 20–25 MB limit) attachments.
• Low engagement rates of the sender.
• Missing sender authentication.
• Spammy language (risk-free, save up to, 100% satisfied, get it now, do it today, etc.).
• Inconsistent sending behavior.

Among all the reasons mentioned above, attachments are among the overlooked. Yet, their impact is huge because they affect the spam score.

Some spam filters use scoring systems to evaluate emails based on multiple factors. The grade is determined by spam filters and is assigned to each individual email. The score is identified based on the different elements of a message, including:

• Content. Filters assess the language to detect trigger words, look out for excessive capitalization, and ensure that the image-to-text ratio is healthy. They also evaluate the links in the text. They shouldn’t be excessive or have problems like being broken.
• Reputation. Algorithms assess the reputation of your domain and IP address by evaluating its sending history and checking engagement.
• Authentication. Filters check for missing or incorrect sender authentication records (SPF, DKIM, or DMARC).
• Technical configuration. Issues like misconfigured server settings, local client problems, connectivity and network failures, and similar technical flaws are detected and considered in your grade.
• Attachment properties. Filters check the number and individual specs of your attachments. Issues like multiple files in one email, too large attachment sizes, risky file types (e.g., exe, .jar, .bat, etc.), naming that doesn’t correspond to the content or triggers spam filters, and similar can also increase the score.

Anything that looks suspicious adds points to your score. Positive characteristics, on the contrary, reduce it. The higher the score, the more likely an email will end up in the spam folder.

As you can see, filters assess several characteristics of an attachment. This means that a single file delivered along with your email can add you two or more spam points, hindering deliverability.

Not always. Adding an attachment doesn’t automatically trigger spam filters. However, it can create additional risk factors depending on the:

• File size
• File type
• Number of attachments

Email systems leverage algorithms that evaluate MIME (Multipurpose Internet Mail Extensions). This is the part of the process aimed at determining whether an email should be classified as spam. Attachments are one of the common MIME types. Algorithms assess their number, size, and types for proper file handling. Some attachment types are safe, while others create risks.

Some types of email attachments are considered safe. This applies to static files. In simple words, safe formats are those that can't contain executable code, macros, or scripts that can potentially carry and install malware, such as:

• PDF (most trusted)
• Images (JPEG, PNG)
• Office files (DOCX, XLSX)

Other file types can be considered unsafe for recipients. This includes:

• ZIP archives
• Executables (.exe)
• Macro-enabled files

So, how to improve email deliverability? By picking the right format in the first place. The best practice is to use clean, lightweight, and predictable files. It’s also important to pay attention to file names. They must be identifiable without opening them.

If you notice your emails going to spam, you can optimize your files and reduce the spam grade. This increases the likelihood of getting into the inbox.

Let’s see how to prevent emails from going to spam in practice:

1. Use Trusted File Formats

Opt for trusted file types. It’s important that both users and systems recognize them. PDF is among the safest ones. Also, it’s safe to use PEG, PNG, DOCX, and XLSX formats.

Risky formats like ZIP or unknown file types should be avoided.

2. Optimize File Size Before Sending

Each email system has its own file size limits. Here are a few common examples:

• Gmail: 25 MB total (attachments + message)
• Yahoo: 25 MB
• Outlook: 20-25 MB
• Others: 15-20 MB

Mind these limits when creating your file to avoid triggering spam filters. What if your attachment exceeds the recommended limit? You can consider replacing it with a link or compressing the file. If you are sending an email with a PDF file, luckily, you can easily compress PDF files with the help of specialized tools. You can also find such tools for other attachment formats.

3. Use Clear, Non-Spammy File Names

File names matter. They must be readable. And users should understand what the file contains from its name. So, make sure to make your names clean and descriptive. Avoid all-caps or odd symbols. And steer clear of spammy language.

4. Limit the Number of Attachments

Too many files attached to a single message look suspicious. They often add points to your spam score. Ideally, limit the number of files to 1-2 per email. 

5. Use Cloud Links When Appropriate

Delivering your attachments via cloud links can reduce attachment-related risk. When appropriate, replace traditional attached files with trusted links. Opt for reputable cloud platforms like:

• Google Drive
• Dropbox
• OneDrive, etc.

These will help increase your trustworthiness, unlike unknown link sources.

Both attachments and links can help you deliver the needed files to the recipient. The primary benefits of links are a lower spam risk (due to the absence of attachments) and easier tracking. However, they aren’t always the optimal way to deliver your files.

When do you use each method? The best approach is to choose based on the type of file. Let’s look at the best use cases for each method.

What can be attached to an email as an attachment:

• Formal documents
• Contracts, reports

What can be attached to an email as a link:

• Large files that exceed recommended size limits
• Multiple files that can be organized as one source
• Marketing materials

Earlier, we’ve detailed how to minimize your spam score. Here’s a quick checklist on how to prepare your attachment and message, keeping email deliverability high:

• Choose a safe attachment format (PDF preferred)
• Reduce file size to recommended limits
• Create a descriptive file name
• Create a clean, non-spammy subject line
• Write clear, non-spammy, and personalized email copy
• Run tests before sending

Here’s an intuitive checklist that compiles the key email deliverability best practices:

Technical Setup

• Configure SPF, DKIM, DMARC

List & Content Quality

• Keep email lists clean and relevant
• Avoid spam-trigger language
• Maintain consistent tone

Sending Behavior

• Always perform email warmup:
  • Start by reaching out to engaged recipients
  • Build your reputation and engagement by delivering quality content
  • Gradually increase sending volume
• Keep your sending frequency and patterns consistent

Engagement Optimization

• Encourage recipients to reply and interact with you
• Monitor your open and reply rates to optimize them when necessary

Attachment Best Practices

• Avoid unnecessary attachments
• Only send safe, relevant, and lightweight files

Apart from following the checklists offered above, keep in mind the following common mistakes that can harm your deliverability:

• Sending an excessive number of attachments
• Sending large attachments repeatedly
• Neglecting email warmup
• Ignoring spam score factors
• Sending important emails from unverified domains
• Overloading your messages with images, links, etc.
• Ignoring personalization

Why are my emails going to spam even without attachments?

Email deliverability is a complex concept. It’s affected by multiple aspects. Apart from attachments, it can be hindered by the sender’s reputation and engagement rate, authentication records, and sending behavior. There is also a list of spam-triggering words.

How to prevent emails from going to spam?

Firstly, send your emails consistently. You need to gradually build a solid sender reputation and stimulate engagement, and then increase the sending volume. When attaching files, choose safe formats and optimize their names and sizes. Also, remember about proper setup, list management, and content quality.

What is a good spam score for emails?

You need to aim for the lowest indicator in spam scoring. A higher grade increases the likelihood of not getting into the inbox. 3 points is a general benchmark. If your score is under it, you should be able to avoid spam filters.

What is the safest file format for email attachments?

PDF is generally considered the optimal format. It’s both safe and familiar to users. PDFs are also easy to manage. You can compress, merge, or split them using convenient tools, which simplifies the process of optimizing your attachments. Other safe file types include images (JPEG, PNG) and office files (DOCX, XLSX).