Lot of emails are being returned with this as reason.
"This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible."
How can I correct this? Thanks.
This is not the reason actually. This text is part of your message. When MaxBulk Mailer generates a MIME message with HTML and a plain text alternative, it also adds this standard message. It is explained here: The Text/HTML format.