Click on the small arrow right beside the "Reply button"
Choose: "Show original"
Is this what you expect

That leads to an another page with out any graphics or any options buttons.
That doesn't solve the issue.The mail content remains the same.

Is the alignment Ok when choosing the option "Message text garbled?" from same menue?

Yes the alignment is ok but it leads to a different window .
There it shows to select encoding type on the browser.None of the encoding type gives a convenient output.
That is the problem.

Comment from David (dhx) mantisbt-help mailing list
My guess is that Gmail is rendering the email in a font that isn't fixed
width. As MantisBT sends plain text emails (not HTML) there isn't really
anything we can do to solve the problem.

Perhaps you're able to set the font in Gmail to "monospace" (CSS value
for the 'font' property) or use a Greasemonkey script in Firefox to hack
around the issue if needed.

basil.mathew, I asked you to check if using "Message text garbled?" shows the alignment you expect. This was the "non technical" way to check what David explained. GMAIL uses a monospcaed font to display the message when clicking "Message text garbled?"