Lucas,

Busca
https://www.google.com/search?q=how+to+convert+an+inline+attachment+email+with+imap+php+to+pdf&client=firefox-b-e&ei=ZWX6YvTJLeOD5OUP_rumiAM&oq=how+to+convert+an+inline+attachment+email+with+imap&gs_lcp=Cgdnd3Mtd2l6EAMYATIFCCEQoAEyBQghEKABOgcIABBHELADOggIIRAeEBYQHToECCEQFToHCCEQoAEQCjoKCCEQHhAPEBYQHUoECEEYAEoECEYYAFCGLVjMRWDUWWgBcAF4AIABtQGIAdUOkgEEMC4xNZgBAKABAcgBCMABAQ&sclient=gws-wiz

imap php
https://www.php.net/manual/pt_BR/function.imap-fetchstructure.php

parece ser pelo ifid que consegue distinguir anexo inline, veja
https://stackoverflow.com/questions/15326329/imap-fetchstructure-how-to-distinguish-embedded-images-from-attachments

fjo