This could be true. There are mail settings both from the server and the receiver. We have done the sender checks that we have done so far using https://www.mail-tester.com/.
Unfortunately, there are no checks we can do on the receiver side. However, there are some good practice protocols that we can follow:
1) On the receiver side you can add our IP address (or domain) to the white-list.
Domain: smtp.shocklogic.com, www5.shocklogic.com
IP Address: 126.96.36.199, 188.8.131.52
We request that your SMTP server administrator add this information to their whitelist.
2) To your domain/DNS you must add an SPF record.
To do this you must log in to your DNS provider and add or modify the SPF text record. If you have an existing SPF record, please add to it, smtp.shocklogic.com and www5.shocklogic.com. Please see here how to add it and SPF record.
3) Each machine receiving emails may have antivirus software installed on it, such as McAfee, AVG, Kaspersky, ect. These antivirus software add an additional security layer for emails being received. Of course we understand some machines will have antivirus software being installed with lower settings and others with higher settings resulting in some emails being received within organisations and others not being received within organisation.
4) If you are using email client like Outlook, please add firstname.lastname@example.org to your contact list.
It is these settings that Shocklogic would like to investigate with yours assistance and collaboration. In order for us to accomplish these successfully, we will need the contact details of the SMTP server administrator. With your authorisation we are happy to reach out to your contact you provide.