Kostya,
Let me share some thoughts on this issue. Your posting of the excerpt from Aquamail manifest got me thinking.
Here is the scenario how what
this_is_nascar described can happen. Namely, how in the absence of the default association set for any mail-related intent in the stock client, the stock client is the only one that is called.
1. This can happen if the stock email client is the only one that has the announced attributes that match those used in the "call" by another app.
2. This attributes are not limited by just the intent. An app can add other parameters (e.g. Type).
I see that what is present in the stock email app manifest (Android 4.1.2 here) and absent in Aquamail and Gmail apps is the following:
<data mimeType="application/eml"/>
<data mimeType="message/rfc822"/>
It is under
<activity name="com.android.email.activity.MessageFileView">
<intent-filter label="@string/app_name">
<action name="android.intent.action.VIEW">
<data mimeType="application/eml"/>
<data mimeType="message/rfc822"/>
<category name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>
So, I am not sure how it would affect the action for android.intent.action.SENDTO or any other SEND* intents. But that seems to be a distinct feature of the stock email manifest, compared to Aquamail and Gmail.
My knowledge of Android, or rather the lack of such does not allow me to complete/check this hypothesis, but I think if you looked at the stock email manifest and read this discussion about using type:
http://stackoverflow.com/questions/2197741/how-can-i-send-emails-from-my-android-applicationit would give sufficient information for you to conclude.
I suspect the browser that this user is using, in addition to the intent might be specifying the mime-type, and that leads to opening the stock email due to the lack of any other app that declared that mime type.
If that's the case, I don't know if you want to change anything in Aquamail, but at least it might provide you something interesting to think about.