We are using Sendgrid, and we have used their SMTP endpoint to send emails. On our end we use PHPMailer to construct the email, and we have emails with embed images (src as CID).<p>PHPMailer and also Swiftmailer sets Content-type to multipart/related when embed images are used. This is RFC 2387 standard. Our emails are displayed fine on every client.<p>We considered to switch to Sendgrid API as it turns out it's faster for us compared to SMTP. We are a high volume client.<p>But, we realized they build the body using multipart/mixed as Content-type. Hence emails on certain clients don't have embed images, instead the embed places are left empty, and there are attached files listed separately. One such client is latest Thunderbird. Other giants Gmail/Yahoo are although displaying emails fine.<p>We contacted Sendgrid support to get explanation why they are using multipart/mixed. At the beginning of the conversation they didn't understood the problem, and they were not on the top of the issue. Then the ticket was escalated to TAM and engineers and they provided just a generic answer, that their system supports only multipart/mixed and they do not consider changing this. No reasoning why they have that.<p>We consider this is a bug.<p>We tried to convince Sendgrid about this bug, but they are sticky on their version. We also asked if this could be configurable via the POST variables, no accept from their end. They even <i>recommended</i> we link or use data uris for our images. We don't want to change this, and I consider these recommendations very cheap, from support personnel. Even more, data uris are not handled well by the email clients.<p>- are we chasing a wrong point?<p>- how can we convince Sendgrid they do this wrong?<p>- how to identify if other clients are displaying embed images wrong? (We know about Litmus, but we need something more as proof).<p>- would you help us to spread the issue, and also submit the same request?
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.
In my oppinion: as long as you pay for a service, that service should support your neeeds; This is a major bug that they were not aware of until now that's why you got a cheap answer.