If message content is not displaying correctly in Postbox, it could be due to the following:
1) Content has been accidentally added to the signature.
By default, Postbox hides signatures in conversation view in order to keep the view clean and easy to read. When signatures are present, some users accidentally place the cursor within the signature and then begin typing. This causes content to be embedded within the signature, and thus, may not be viewable in Postbox's conversation view.
To test if this is the case, simply open the message in a new tab or window, as Postbox does not hide signature content in stand-alone view. If the content then displays, you'll know that content has mistakenly been added to the signature region.
To prevent this from happening in the future, we recommend setting your signature text to a different color than your body text, or by creating top and bottom borders to your signature, as demonstrated in this help article:
Additionally, you can turn off the option to hide signatures in Preferences | Options > Display > Conversations > Hide Signatures.
2) The content you've received is actually an attachment
For reasons unknown to thinking people, some content producers (e.g. newsletter authors) embed content within a MIME attachment. If you have Postbox set to not display attachments inline, then you will not be able to view the content.
To address this, you can go to the "View" menu and select "Display Attachments Inline."