I have tried everything I could think of.
I think something went wrong with the upgrade to 10.1 php, horde or some other issue that I have not been able to figure out.
At the moment I have found a small solution, which is to edit html in the signature section of horde as I indicate in the screenshot that I leave in the address:
http://ayuda.seycob.es/smeserver-horde.pngIn this case the image does not load and is not visible in the email, but the recipient receives the signature correctly.
Signature urls are resolved both externally correctly and internally from smeserver, but apparently not from horde.
Another problem is that when I create a user's email address I create it as user@correo.seycob.es and not as user@seycob.es
Thank you for your time and responses.