After changing the configuration of the mail server, mails received from a specific sender are garbled.

Issue

The pipeline which our emails go through has changed, our emails are now garbled/not arriving properly any more. These emails have glyphs which are not in the ASCII specification, Japanese kanji for example. Our initial mail chain:

    ---------------
    Internet
    ---------------
       |
    ---------------
    FW
    ---------------
       |
    ---------------------
    Linux server(postfix)                <-- first mail server (rhel7)
    ---------------------
       |
    ---------------
    FW
    ---------------
       |                                             |
    ----------------------------------      ----------------------------------
    Scanner1(SymantecMessagingGateway)      Scanner2(SymantecMessagingGateway)
    ----------------------------------      ----------------------------------
       |                                             |
    ----------------------
    Linux server(sendmail)               <-- second mail server (rhel7)
    ----------------------
       |
    ---------------
    core L3 switch 
    ---------------
       |
    ---------------------------
    Each department mail server
    ---------------------------

With the change, the Scanner-systems were removed, and postfix is now directly delivering to the sendmail system.

We noticed some differences on the sendmail system after the change:

• Mails are logged with "bodytype=7BIT" instead of "bodytype=8BITMIME" which was logged before
• in the header of the mails, "Content-Transfer-Encoding: base64" changed to "Content-Transfer-Encoding: 7bit" due to the change.

When we sniffed traffic, we saw that the postfix system already is getting the problematic mails with "bodytype=7BIT" - for mails which have clearly more than ASCII characters.

How to fix this issue?