One of the following situations could be occuring:
- When a message comes across as one of the following: garbled text, a mix of plain text and HTML code or just the plain text, you or your recipient's spam filter has flagged your email. The spam filter or email server scanning your incoming message has seen something it doesn't like in your content and has "quarantined" the message. This creates the format changes you're seeing. And, this situation will happen to everyone who receives that email in the same email environment.
The only people that can determine why your subscribers are seeing a broken email is the IT group that manages your subscriber's incoming email servers. - In a similar way, anti-virus programs will sometimes "break" multipart email templates and display them as raw HTML code. These kind of issues are generally very isolated.
To fix this, you'll need to change your anti-virus settings to accept or trust multipart email messages. For additional guidance, please contact the software company that produces your anti-virus program. - Some email programs will display multipart messages as garbled code or an attachment, if an email message doesn't contain both HTML and plain text content. You can fix this problem by adding either plain text or HTML content if one, or the other, is missing.
To test if this is the problem, try sending your test from the last step of the campaign builder instead of from the HTML or plain text creation steps. - If you are sending to MS Outlook accounts, you may want to try removing Merge tag personalization from the subject line. Learn more about the subject line personalization and Outlook here.