When an email is being sent, the receiving server checks, wether the sending server is permitted tho send from the given email-adress. If this isn't the case, the mail will be rejected, depending on the server-side settings.
This can be avoided by adjusting the so-called SPF-record of the server in whose name the mail is being set.
Please contact your system administration and as them make the according changes.
The SPF-record for our mail servers is:
For additional information regarding SPF, please read the related article at:
These changes will not necessarily solve the problem you are experiencing, as there are numerous factors that might be causing this behaviour.
Adjusting the SPF-record merely eliminates one of these factors.