What attachment types aren't supported?

Attachments are supported on POST /emails, but two things commonly cause failures: size limits and blocked file types. Both matter because even an attachment MailBlastr accepts can be stripped or quarantined by the receiving mail provider.

Size limits

MailBlastr enforces these caps and rejects the send if either is exceeded:

These apply whether you supply the file inline as base64 content or as a hosted path URL that MailBlastr fetches. For a path, an oversized file is rejected up front using its Content-Length, and again after download if the actual size exceeds the cap.

Blocked / risky file types

There is no benefit to attaching an executable or script file — virtually every major provider (Gmail, Outlook/Microsoft 365, Yahoo, and corporate gateways) blocks or quarantines these regardless of what the sending platform allows. Common examples that are routinely rejected:

• Executables and installers — .exe, .msi, .bat, .cmd, .com, .scr, .app, .dmg
• Scripts — .js, .vbs, .ps1, .sh, .jar, .py
• Office documents with macros — .docm, .xlsm, .pptm
• Archives that hide the above — .zip / .rar / .7z containing blocked types are often stripped too

Files with no declared or recognizable MIME type fall back to application/octet-stream, which spam filters tend to treat with extra suspicion — so even an otherwise-fine binary can hurt deliverability.

Full list of blocked send extensions

For completeness, the following extensions are blocked for sending (you can still receive any of them). The list is the standard set of executable, script, and dangerous types rejected across providers:

Recommended: link, don't attach

For anything large, risky, or executable, host the file and send a link instead of attaching it. This keeps you under size limits, avoids type-based blocking, lets you update or revoke the file later, and improves deliverability.

1. 1
   Host the file

   Upload to your own storage (S3, a CDN, a file-sharing service) and get a stable URL.

2. 2
   Link in the email body

   Include a clear download link in the HTML instead of an attachment.

3. 3
   For supported attachments, use a clean type

   PDFs, images (PNG/JPG), and CSV/plain-text files are safe to attach within the size caps.