How to Decode an Email in MIME Format

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Most emails sent today are MIME (Multipupose Internet Mail Extensions) formatted. This allows emails to be sent with plain text and rich text/HTML versions, inline images, and attachments. MIME extensions can be added to a message in standard RFC/822 format so backward compatibility is achieved with older mail systems.


Step 1

Load the contents of the email message.

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Step 2

Check the "Content-Type" header. If the content type is multipart (i.e. "multipart/mixed", "multipart/alternative", etc) the message will have multiple sections to parse. If the content type is multipart, continue with below steps.


Step 3

The Content-Type header should contain a unique string used by the code that generated the MIME message to designate MIME part boundaries. Example:

Content-Type: multipart/mixed; boundary="part_c7161025_fe8a_45f4_83ef_6befcfa5d021"


The boundary string in this case is "part_c7161025_fe8a_45f4_83ef_6befcfa5d021".

Step 4

Each MIME part will be separated by the boundary string preceeded by two dashes ("--") on a blank line. You will need to scan the message line by line looking for the part boundaries. All text in between the boundaries except the final CR/LF is part of that MIME part.


Important: The last MIME part will end with the boundary string followed by two more dashes.



--part_c7161025_fe8a_45f4_83ef_6befcfa5d021 First MIME part.


--part_c7161025_fe8a_45f4_83ef_6befcfa5d021 Second MIME part

--part_c7161025_fe8a_45f4_83ef_6befcfa5d021 Last MIME part.



Step 5

Each MIME part then needs to be parsed. The format of each MIME part is similar to the original RFC/822 formatted message. It will contain a series of headers, followed by a blank line, then the body data.

Step 6

A MIME part can itself be multipart, with its own "Content-Type" header and boundary string. Your code must recursively parse MIME parts until it reaches the child nodes.


Step 7

A MIME part can simply be a binary attachment. In this case a Filename header will usually be supplied, along with the transfer encoding. BASE64 is a popular encoding type. In this case the entire MIME part body must be BASE64 decoded. As an example here are the first few lines of an attached JPG image:



------=NextPart_003_01C755EF.43F2628D Content-Type: image/jpeg; name="testimage.jpg" Content-Transfer-Encoding: base64 Content-Description: testimage.jpg Content-Disposition: attachment; filename="testimage.jpg"



Step 8

The HTML body of the message should be in a MIME section with ContentType: Text/Html, inside a ContentType: Multipart/Alternative MIME section.

Step 9

The plaintext body of the message should be in a MIME section with ContentType: Text/plain, inside a ContentType: Multipart/Alternative MIME section.



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