Dear Dr. Durand,
problem with this topic is, that there's tons of potential sources for these kind of Problems and the vast majority of these doesn't lie within our reach.
The first thing we usually advise on regarding this matter, is contacting the IT department responsible for the toplevel domain your email-adress is using. In most cases those emails are connected to a university, like firstname.lastname@example.org
The IT department of exampleuniversity.com should thus be contacted in order to add our servers to their SPF entry:
Further Information on SPF in general: