Office process various WordPress based forms as Formidable Pro Plugin which send email receipts. Sometimes, an email receipt can come through corrupted– equal sign along with broken HTML that will appear in the body part of email. However, such happens due to messages received by Outlook clients which is mostly used by everyone for their official purpose.
For more knowledge visit- https://wordpress.org/support/topic/wp-mail-outlook-mime-multipart-issues#post-7670292
MS Outlook is mainly used as an email application. It contain a calendar, task manager, contact manager, note taking, journal and web browsing. All these information is stored in PST file. User can access information from PST file. But sometimes there is problem arises when you want to access data from PST file. The reason behind accessing problem is PST file corruption.
Whatever, in above case sometimes, when viewing the source in Outlook email there may be following errors: “This message is in MIME format. Since mail reader has no idea about this format, some or all the messages may not be legible.” However, such condition occurs and creates more confusion and also very difficult to address by the the given below details:
-This doesn’t display a specific problem for Formidable Pro. Other Plugins which send mail via WordPress are also affected.
-Every email is not affected, Person A’s form submission email may come through fine, Person B’s form submission email has corrupted– with seemingly no pattern
Mail coming from non-WordPress or other applications on the same web server, using same SMPT to send mail are not affected.
For Further Information – https://zorebenezer.wordpress.com/tag/outlook
However, if Outlook corruption occurs, then there is also solution available which has inbuilt utility named Scanpst.exe or Inbox Repair Tool but it works at an extent limit. In most of the cases this tool is not working properly. Tool having some limitation under which it works. If your PST file is larger than 2 GB then ScanPST.exe is not able to repair PST file. Inbox Repair tool is not able to repair complex PST file. In such condition, PST Repair Tool, this is the most reliable tool in Outlook segment. This tool is best for repair corrupted PST file Successfully. It can handle all PST corruption like PST header corruption, broken PST file, virus attack, over sized data etc.
Apart from that, You should be think about your WordPress security, you should never activate vulnerable Plugin which may cause lots of issues and also further may cause of malware attacks, and lots of damaging effects.