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Editorial Team
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Asked: 2026-05-14T04:36:34+00:00

From reading the RFC it appears that CID can/must only contain characters from the

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From reading the RFC it appears that CID can/must only contain characters from the same set as those permissable by a regular URI. Is this correct. Im asking because I wish to writeup a simple helper that takes a CIDs prefix and adds a counter when generating CID for mime multitypes attachments.

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  1. Editorial Team

    The Content-ID value is required to be in the form of an RFC-822 addr-spec (user@domain).

    Since an addr-spec may contain characters not allowed in a URL, those characters are hex-encoded when used in the “cid” URL.

    Per RFC2392:

    A “cid” URL is converted to the
    corresponding Content-ID message
    header [MIME] by
    removing the “cid:” prefix, converting the % encoded character to
    their equivalent
    US-ASCII characters, and enclosing the remaining parts with an angle
    bracket pair,
    “<” and “>”.

    Read more: http://www.faqs.org/rfcs/rfc2392.html

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