In your position I would be looking carefully at the HTML code of the two emails you sent to yourself. (View Source will reveal it.) See if there are font definitions, font size or styling tags that differ between the two messages. Once you have identified what is causing them to appear differently you can address the cause. Also I would advise closing and restarting Thunderbird with add-ons disabled and repeating the experiment, in case an add-on is the trouble.

Im code illiterate unforunately

Here are the codes

!st email:

x-store-info: Ru8Mzrcu9Bi87krfImffoGU6zyI7zQxEX0BuL8ZH8bGeW4+sTAXtk75KBNMzrywMGLDM2u+BSScXHP/bRwoNJ7SscHUtRO9iBVVm/NRhO5VCOHe6NIjSI8itII1jeWYFrfmKVbuGppOr5wLwNmKa3A== Authentication-Results: hotmail.com; spf=pass (sender IP is 65.55.116.43; identity alignment result is pass and alignment mode is relaxed) smtp.mailfrom=jeniva.yeo@hotmail.com; dkim=none (identity alignment result is pass and alignment mode is relaxed) header.d=hotmail.com; x-hmca=pass header.id=jeniva.yeo@hotmail.com X-SID-PRA: jeniva.yeo@hotmail.com X-AUTH-Result: PASS X-SID-Result: PASS X-Message-Status: n:n X-Message-Delivery: Vj0xLjE7dXM9MTtsPTE7YT0wO0Q9MDtHRD0wO1NDTD0w X-Message-Info: AuEzbeVr9u6HQKaFYM906NxwDv8EUrWGfwry/47sT6fJKamxRlmRSrVcJrhI8WnxQgnIK4efTPls3kHusg9nDs+t2zhn/1eer2G9RqhFjfBXp/nizPLuLeqvSjzjUlv1FmoEqJ2oKT3zcgmOE8nIQXGZGsq1OLhyT/znyybB0PyTjq/9kvj11TWMXhPEGlukZl4lZEKzitE7f/hHk8tLEQH6GkCd6KwYCrIjk3ifKojWZftGn64V4A== Received: from BLU004-OMC1S32.hotmail.com ([65.55.116.43]) by COL004-MC5F18.hotmail.com over TLS secured channel with Microsoft SMTPSVC(7.5.7601.23143); Wed, 13 Apr 2016 22:08:48 -0700 Received: from BLU436-SMTP40 ([65.55.116.8]) by BLU004-OMC1S32.hotmail.com over TLS secured channel with Microsoft SMTPSVC(7.5.7601.23008); Wed, 13 Apr 2016 22:08:41 -0700 X-TMN: [plDXeOgh3acNWKZFoFAPFcuoyVBEBw3N] X-Originating-Email: [jeniva.yeo@hotmail.com] Message-ID: <BLU436-SMTP409FAA2E5FA5E08B51F6638D970@phx.gbl> Return-Path: jeniva.yeo@hotmail.com To: jeniva.yeo@hotmail.com From: Jeniva Yeo <jeniva.yeo@hotmail.com> Date: Thu, 14 Apr 2016 13:08:35 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101

Thunderbird/38.7.2

MIME-Version: 1.0 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 14 Apr 2016 05:08:40.0107 (UTC) FILETIME=[B54E4FB0:01D1960B]

<meta http-equiv="content-type" content="text/html; charset=utf-8"> <font size="+2"><font face="Arial">test</font></font>

-- 
Warm regards
Jeniva Yeo

2nd Email :

x-store-info: Ru8Mzrcu9Bi87krfImffoGU6zyI7zQxEX0BuL8ZH8bE2GaBRHV8Y1mFF1e00lpaMMpijQhWrQ2Kw2a5/SEgvBhsWE9G6FkF8U6gwrY7+QRoi+yAODecib0Na6IDQhXpRtpaMWOdwHcXPvKae8V2iXg== Authentication-Results: hotmail.com; spf=pass (sender IP is 65.55.116.28; identity alignment result is pass and alignment mode is relaxed) smtp.mailfrom=jeniva.yeo@hotmail.com; dkim=none (identity alignment result is pass and alignment mode is relaxed) header.d=hotmail.com; x-hmca=pass header.id=jeniva.yeo@hotmail.com X-SID-PRA: jeniva.yeo@hotmail.com X-AUTH-Result: PASS X-SID-Result: PASS X-Message-Status: n:n X-Message-Delivery: Vj0xLjE7dXM9MTtsPTE7YT0wO0Q9MDtHRD0wO1NDTD0w X-Message-Info: AuEzbeVr9u6HQKaFYM906EYSN7oPsRu9r67svucJCVJ5VAD24UkP4qSIghJ4uPYSBRmwqm0W6nXOY8MgNNpeozN/aASJBKl27FX5JSAgRGGMwpKVsSyr8CyHXuJ0g8tnjCZj87Il9i2cA0jK2EGAHBn1w+hitYtELuL0/dEkbJIbRE+tr86cKYNGIF2eR6mTzOrUKtameucITaNahsAsztZ0s1AyT0wX6H090oc182nFGOh5qS5ZFQ== Received: from BLU004-OMC1S17.hotmail.com ([65.55.116.28]) by COL004-MC4F32.hotmail.com over TLS secured channel with Microsoft SMTPSVC(7.5.7601.23143); Wed, 13 Apr 2016 22:07:34 -0700 Received: from BLU436-SMTP248 ([65.55.116.7]) by BLU004-OMC1S17.hotmail.com over TLS secured channel with Microsoft SMTPSVC(7.5.7601.23008); Wed, 13 Apr 2016 22:07:25 -0700 X-TMN: [ZmS7iM1X+BSHUE1FAGJC8U8J99fisTbR] X-Originating-Email: [jeniva.yeo@hotmail.com] Message-ID: <BLU436-SMTP2480E9E59808CD161E972248D970@phx.gbl> Return-Path: jeniva.yeo@hotmail.com To: jeniva.yeo@hotmail.com From: Jeniva Yeo <jeniva.yeo@hotmail.com> Date: Thu, 14 Apr 2016 13:07:19 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101

Thunderbird/38.7.2

MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 14 Apr 2016 05:07:24.0898 (UTC) FILETIME=[887A5420:01D1960B]

test

-- Warm regards Jeniva Yeo

Well the first one appears to have been composed and sent as html and the second one as plain text. I'd suggest: 1) in Account Settings > Composition & Addressing, make sure that 'compose messages in HTML format' at the top is ticked. 2) in Preferences > Composition > General, make sure that the font size for HTML is set to 'medium' 3) in that same control, click on 'send options' to 'configure text format behaviour' and make sure that 'send in HTML anyway' is selected 4) close and restart Thunderbird to make sure that those changes have taken effect and then 5) when composing an email, click on Options and check that Delivery Format is set to HTML With those steps in place, repeat the experiment: is the appearance of the messages when you receive them now consistent? Please note that those steps are to ensure that HTML is used when you compose messages. You also need to be thinking about how the messages display when opened. You control that also in Thunderbird Preferences, on the Display > Formatting tab. Personally I have the default font for HTML set to sans serif size 14 and plain text set to regular font and regular size.