What 550 5.7.1 Means
Gmail rejected your message because it contained an attachment of a type Gmail considers high-risk. Per Gmail's policy, executable files (.exe, .bat, .cmd, .vbs, .scr) and certain archive formats are blocked unconditionally because they're the primary vehicle for malware delivery. The full text typically reads: 'Our system detected an illegal attachment on your message.'
Gmail consumer accounts and Google Workspace domains. The list of blocked file types is consistent across Google's infrastructure.
Your message has an attachment in a blocked file type. Common offenders for legitimate senders: .exe installers, .bat utility scripts, .iso disk images, .zip files containing blocked types, or password-protected archives that Gmail can't scan.
How to Fix 550 5.7.1
- 1
Remove the executable attachment
Cold email essentially never needs an executable attachment. If you're attaching .exe, .bat, .vbs, or similar, host the file at a download URL and link to it instead. Cloud-hosted (S3, Drive, Dropbox) with direct link is the clean approach.
- 2
Avoid archive formats Gmail can't scan
Gmail can scan .zip and .tar contents. It cannot scan password-protected archives or some encrypted formats. Password-protected attachments are blocked unconditionally. Use unprotected archives or link to cloud-hosted files.
- 3
Check the full list of Gmail's blocked file types
Gmail publishes the blocked-types list at the link in references. Common surprises: .js, .jse, .vbs, .vbe, .ps1, .ps2 (script files); .lib, .dll (system libraries); .ade, .adp (Access projects). If you're attaching any of these, switch to a link.
- 4
Don't try to bypass with renamed extensions
Gmail scans file content, not just extension. Renaming .exe to .txt doesn't bypass the scan. Some senders try double-extensions (.txt.exe) — Gmail catches these too. The clean fix is hosting the file remotely.
- 5
Use Drive / Dropbox / S3 + a link
Host the file on a sharing service and embed the public link in your message. Gmail's filter is more lenient on links to known file-hosting services than on direct attachments. Use a tracking-friendly domain if you need download analytics.
References
- ◇Gmail Help — Attachments You Can't Send
Full list of blocked file types in Gmail.
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550 5.7.1 in the Cold Email Context
Attachment rejections are rare in cold email because cold email rarely needs attachments. The typical case is a salesperson attaching a PDF deck or proposal — these are normally accepted (PDF isn't blocked). The case that does fail: any attempt to attach an installer, script, or password-protected archive. The deliverability-safe pattern for cold email is text-only with a link to anything heavier (case study PDF, deck, demo video). ColdRelay infrastructure doesn't change this — it's a content-layer issue handled by your sending platform.
Frequently Asked Questions
Is PDF blocked by Gmail?
No. PDF is allowed. Standard document formats (PDF, DOCX, XLSX, PPTX, images) are all accepted. The blocked list is specifically executables and risky scripts.
What about ZIP files?
Unencrypted ZIPs are accepted IF their contents don't contain blocked file types. A ZIP containing an .exe is blocked because Gmail scans archive contents. A ZIP of PDFs is fine.
Why are password-protected archives blocked?
Gmail can't scan the contents, so they can't verify the archive doesn't contain malware. Password-protected archives are a common malware delivery vehicle, so Gmail blocks them by default.
Will sending the file via Drive bypass the block?
Yes for the email itself (no attachment = no rejection). But Drive sharing has its own restrictions on what file types can be hosted and shared with anonymous viewers. Most non-executable files are fine on Drive.