I faced the problem in outlook web application and Zimbra's mail web application too. If I manually include image and send it to these services, then it works completely fine. But if i use Mail dll to send embed image, as mentioned in the instuctions,it doesnot work for others but gmail only.
And also, the instruction in the link from my question uses
builder.AddVisual. Not, IMail.AddVisual. Is that causing the issue?