When you search about this issue, you will find several topics, all stating the multipart/mixed is the trouble, and multipart/related should be used for embeddi

When you search about this issue, you will find several topics, all stating the multipart/mixed is the trouble, and multipart/related should be used for embedding images. Swiftmailer has a unit test just to check this. We reported several articles/topics which all had this problem, and it went away switching to multipart/related.