Announce list question: multiple spaces -> tabs?

In mailouts using Dreamhost’s Announce List feature, I sometimes use multiple spaces for formatting purposes in the body of the message. Occasionally though, once the message is sent out and received, certain (but not all) instances of multiple spaces will be apparently be interpreted as a tab character. This generally disrupts the intended formatting, and deep-sixes my attempts to appear professional, consistent, and detail oriented (read: impossibly anal retentive).

Messages being sent are plain text (i.e. HTML isn’t involved). Cases where this happens seem really inconsistent. If there’s a pattern (i.e. quantity of consecutive spaces), I can’t find it. Any idea why this happens, and more importantly, how to prevent it?

Thanks in advance.