[ A typical simple HTML mail created using Tb's HTML composition mode ] [1] Actions at Composer of Tb. In Body Text(or Paragraph, or Preformat) use Variable Width and Fixed Width(tt> is currently used) Copy a link at text mail display of SeaMonkey, then Paste at Body Text of Tb's Composer. Add Paragraph(

) and Preformat(

).
  Insert Link, Inser Table, Insert Image are not used.
  Insert HTML is not used.
  Any text formatting is not used(bold, italic, underscore, font, color, ...)

(Created HTML)


  
    
  
  
    VariableWidthFont

    FixedWidthFont

    
https://bugzilla.mozilla.org/show_bug.cgi?id=414299

    
The quick brown fox jumps over the lazy dog.

      The quick brown fox jumps over the lazy dog.

    

    
The quick brown fox jumped over the lazy dogs.
The quick brown fox jumped over the lazy dogs.
 

  



[2] If this kind of HTML, known phenomenon of bug 414299 occurs.
    If Delivery Format=Auto-Detect,
    and if recipient is Prefers=Unknown mail address,
    this HTML mail is silently sent by Tb with unknown reason.

This is perhaps "Data Loss!!!", "Format Loss!!!" which is seen in bug 414299,
and is seen in recent some bugs such as Bug 1204379.

Note:
(1) In HTML mail composition, following is needed to see phenomenon.
  At Tools/Optins/Composition, General, HTML :
     Click [Restore Defaults]
       Font: [Variable Width] can be changed to [Fixed Widrh] becuse:
         Variable Width =  is not used for test
         Fixed    Widrh =  is used for test
       Size: shouldn't be modified.
         If modified,  is used, so phenomenon disappears.
       Color: shouldn't be modified.
         If modified, different color value is set in next tag,
         so phenomenon disappears.
            
            (default is )
(2) Recipient's Email Address should be:
    Domain of Email Address is not regitered to HTML Domain/Text Domain.
    And one of next.
      - A Contact in Address Book with Prefers=Unknown
      - Email address which is not registered to Address Book

[3] Following is text/plain mail data sent by Tb.

VariableWidthFont
FixedWidthFont

https://bugzilla.mozilla.org/show_bug.cgi?id=414299

The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy dog.

The quick brown fox jumped over the lazy dogs.
The quick brown fox jumped over the lazy dogs.

[4] CSS Style case.
Following is simple HTML example with CSS Style in Bug 584313.
This kind of HTML with CSS Style is also considerd "Converible to text",
so, if unknown recipient, mail is currently sent as text/plain by auto-downgrade.






Type text here
    

      pppppppppppppppppppppppppppppppp
      

    




I don't know how to force "