Have you already tried using an EmailAdapter instead of a MailEnvelope?

You could create an email submit data connection via InfoPath's user
interface, then retrieve it in code and use it as an EmailAdapter to submit
your form. The EmailAdapter object offers more flexibility/possibilities than
the MailEnvelope. See
http://msdn.microsoft.com/library/en-us/ipsdk/html/xdobjEmailAdapterObject_HV011
06499.asp?frame=true
---
S.Y.M. Wong-A-Ton

Hi Scott

To use this code, you will first have to add E-mail as a Submitting Data
Source.

1) In Design mode, choose Data Connections from the Tools Menu and then
click Add.
2) Select Submit Data, and then click Next.
3) Select as an e-mail message, and then click Next.
4) Delete all text in all textboxes on the window (you will be adding all of
this content back in later using code). Click Next.
5) On the next window enter a name into the textbox. Remember what you name
it. Click Finish.

U can write code as :

CTRL136_7::OnClick(eventObj)
- XDocument::OnLoad(eventObj)
- CTRL211_7::OnClick(eventObj)
- XDocument::OnSaveRequest(eventObj)
- XDocument::OnSign(eventObj)

Write this code in above functions any one.

var objEmailAdapter;
objEmailAdapter = XDocument.DataAdapters("THE NAME YOU TYPED IN STEP FIVE");
objEmailAdapter.To = "list@example.com";
objEmailAdapter.Subject = "Status Report";
objEmailAdapter.Submit();