If I send a graphic passcode (CAPTCHA image) but the recipient reads all e-mails in text only mode, will they have an attachment that they can open to see the graphic image? Attachments can have either disposition of attached or inline. They are always within the body of the e-mail as a MIME part but the disposition dictates whether the attachment should be presented by an e-mail client as outside the body (say with a paperclip icon) or within the body. If they are inline but the recipient reads in plain-text only mode then I’m wondering if they will also see the attachment separately so they can look at the image.
I would like to send the passcode as a CAPTCHA image. I suspect that also means that it will force the challenge e-mail to be HTML formatted (to show the image inline of the body of the message). I’m not sure that users that read their e-mails in plain-text only mode that receive HTML-formatted e-mails will be able to see the image. HTML-formatted e-mails, by standards, are supposed to include both a plain-text and HTML MIME part (i.e., they double up on the content by including both versions within the message). Hopefully Comodo follows the standards by including both a text and HTML MIME part within an HTML-formatted e-mail; otherwise, plain-text reading recipients won’t be able to read the challenge e-mail. The plain-text version will obviously not display the image within the message. Hopefully it will still be presented as an attachment that the recipient can open to see what passcode they are supposed to use in their reply.