Since a few days ago, messages from the server correo.efismart.es (126.96.36.199) are not arriving to destinations managed by you. As an example, legitimate mails sent from firstname.lastname@example.org to email@example.com do not reach destination, the mails are accepted by destination servers but they not appear in the recipient mail, so I undertand that the mail is silently rejected. The domain montajesfiver.com is managed by the correo.efismart.es server.
I have revised the mail server logs of the last few days and I do not find any suspicious activity, only the usual mail movement. Also I have check the mail address agaisnt several reputation services and all of them return that the reputation is good or neutral, also the ip is no listed in any blacklist service but yours.
The IP of the server appears with negative reputation in https://ipremoval.sms.symantec.com/. I tried to send an investigation request but the form keeps show up again and again when I press the investigate submit button, so I am not sure that the request is sent.
Any help will be appreciated.