I was messing around with OpenPop, a library written in C# for POP3 mail fetch. It seems to work OK, but I didn't quite get the idea how to make a difference between explicitly attached files to mail, and stuff like pictures in HTML-content mails. This library treats them all as "attachments". For my needs I wouldn't consider a picture within HTML mail an attachment.
From the library docs:
A MessagePart is considered to be an attachment, if
- it is not holding text and is not a MultiPart message or
- it has a Content-Disposition header that says it is an attachment
What should I do or search for, at least in theoretical terms (because I'm not really familiar with mail protocols)?
You could check if the image file is referenced in the body text of the email. You would need to parse the HTML and look for tags such as img or the background-image property in a CSS selector. If the image is not used by the message itself, then consider it to be a "genuine" attachment.