MIME Boundary Generator
The boundary string in multipart MIME must not appear in any encoded part. This tool generates one that's safe.
Record generated. Now make sure it actually passes on real sends.
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Verify on a real sendGenerates a 32-character random boundary with a recognizable prefix. Returns the full Content-Type header and the boundary delimiter lines.
Building MIME messages by hand. Debugging a 'boundary appears in body' MIME parser error.
FAQ
What makes a boundary string safe?+
It has to be a sequence that never appears anywhere in the encoded parts of the message. This generator uses a fixed prefix plus 24 random characters, which is long enough that an accidental match in normal body content is not realistic.
Does it check my message body for a collision?+
No. It generates the string and prints the multipart skeleton, but it never sees your actual parts. If you're debugging a boundary appears in body error, search your encoded parts for the new string once to confirm.
Is this random enough to use as a security token?+
No, and it isn't meant to be. It's ordinary browser randomness, which is right for a MIME delimiter and wrong for anything that needs to be unguessable, like a session ID or a password reset token.
Keep going
Articles, glossary entries, and other tools on the same topic.
Nate built Outsolvi after watching every email-tracking tool he had ever used lie to him about opens. Outsolvi runs Tier 1 to 5 confidence scoring on every open, native in Outlook and Gmail, so the number on the dashboard is one a rep can actually act on.
We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.