Attachment

TL;DR: Attachments let you share files through email instead of just text. They're subject to size limits, can trigger security filters, and sometimes cause deliverability issues if handled poorly.

Why attachments changed everything

Email was originally just text. Attachments made it actually useful for work.

You need to send contracts, share photos, distribute reports. Typing everything into the email body isn't practical. Attachments solve this by encoding files into a format that email servers can handle (called MIME encoding) and delivering them alongside your message.

But here's the problem: attachments are a security nightmare. According to Verizon's Data Breach Investigations Report, 96% of phishing attacks arrive by email, and malicious attachments are a primary vector. That Word doc could contain malware. That PDF might be designed to steal credentials. This is why spam filters scrutinize attachments heavily and sometimes block legitimate files.

Size is the other big issue. Most email servers cap attachments at 25MB (Gmail and Outlook both use this limit). Trying to send something bigger usually fails, bounces back, or forces you to use cloud links instead. And even if your server allows larger files, the recipient's server might not. 

The shift now is toward sharing links instead of actual attachments. Cloud services like Google Drive, Dropbox, or OneDrive let you send a link to the file rather than embedding it in the email. Smaller message size, no server limits, better version control. Still technically an attachment workflow, just smarter.

Different attachment types

Traditional attachments are files you actually include with the email. They get encoded, sent through SMTP, and downloaded by the recipient. These count toward message size limits and increase email deliverability risk if they're too large or the wrong file type.

Cloud-based attachments (or "link sharing") don't actually attach the file. Your email client uploads it to your cloud storage and inserts a shareable link. Gmail calls these "Drive links." Outlook uses OneDrive. The recipient clicks the link and accesses the file in their browser. Way more reliable for large files.

Inline attachments are images or files embedded directly in the message body, not listed as separate downloads. Like when you paste a screenshot into an email and it just shows up in the text. Technically still an attachment, but displayed differently. Often used in HTML email signatures or formatted messages.

Some systems also differentiate between regular attachments (which anyone can see) and encrypted attachments (which require a password or certificate to open, used in S/MIME setups). The latter is rare outside highly regulated industries.

Adding files to your emails

The process is straightforward, but options vary by client.

In Gmail:

• Click Compose to start a new email
• Hit the paperclip icon at the bottom of the compose window
• Choose Upload to attach from your computer
• Or click the Google Drive icon to attach from Drive (this creates a link, not a traditional attachment)
• Select your file and it'll upload automatically

Gmail shows a progress bar while uploading. If you attach something over 25MB, it automatically converts it to a Drive link instead.

In Outlook:

• Open a new message
• Click the paperclip icon (Attach a file to this) in the ribbon (or Insert > Attach File)
• Choose Browse This Computer for local files
• Or select Upload and share, or select OneDrive for OneDrive files
• Pick your file and click upload

Outlook also lets you attach emails as files, which is weirdly useful for forwarding multiple messages as a single package.

In Spark (macOS):

• While writing or replying to an email, click the paperclip at the top right.
• Click Attachment and choose the files you wish to attach. To select multiple files at once, hold Cmd.
• Click Open.
• Note: The total size of the attachments should be less than 25 MB. If your files are larger, we recommend you upload them to cloud storage and paste the link into your email. Alternatively, you can use Spark Cloud on a mobile device to upload large attachments to an email.

Find out in more detail how to add an attachment to an email with Spark or to add an attachment on different platforms on our dedicated help page: https://sparkmailapp.com/help/sending-emails/attach-a-file-to-an-email

Do's and don'ts for attachments

Check the file size before sending. If it's over 10MB, seriously consider using a cloud link instead. Large attachments slow down sending, take forever for the recipient to download, and often get blocked by security filters. Under 5MB is ideal.

Zip multiple files. Sending seven separate attachments looks messy and increases the chances of someone missing one. Compress them into a single .zip file. Cleaner, smaller, easier to manage.

Use descriptive filenames. Don't attach "Document1.pdf" or "IMG_2847.jpg." Rename it to something useful like "Q4_Budget_Proposal.pdf" or "Office_Floor_Plan.jpg." Your recipient will thank you.

Scan for viruses first. Even if you're 100% sure your file is safe, run a quick virus scan before sending. Your recipient's company might block it if their security software flags it, and you'll look careless if you accidentally send malware.

Avoid executable files. .exe, .bat, .com, .scr. These file types get blocked by almost every corporate email system because they're common malware delivery methods. If you absolutely must send executable files, zip them with a password and send the password separately.

Warn people about large attachments. If you're sending something close to the 25MB limit, give the recipient a heads up. "Sending you the full video file, it's about 20MB" in the email body sets expectations and prevents them from thinking the email is broken when it takes a while to download.

Don't use attachments for sensitive information. Unencrypted attachments aren't secure. If you're sending financial data, personal information, or confidential documents, use end-to-end encryption or a secure file-sharing service with access controls.

Related content

• Email Etiquette Rules for Professionals

Related terms

• MIME Type
• Phishing Email
• Email Deliverability
• Spam Filter