Some mail servers, such as gmail.com, use the so-called Sender Policy Framework (SPF) to defend against spam. The principle of SPF is that the recipient of the letter (in your case, for example, gmail.com) checks whether the address of the sender of the letter matches the address of the computer from which the letter was actually sent. If it does not match, the recipient may reject the email as undeliverable.
Tip: ExampleHere's an example: The user xxx@mail.ambis.cz set up in IS AMBIS to redirect mail to
xxx@gmail.com. When e-mails are sent to user xxx@mail.ambis.cz from eg mail-services.com, it may
reject gmail.com's e-mails, because an email that appears to have been sent from mail-services.com actually came from mail.ambis.cz.
Because the user has forwarded mail from IS AMBIS.
If you have redirected letters from IS AMBIS and the letters return to you as undeliverable,
the solution is either to cancel the redirection of mail from IS AMBIS or to change
the addressee of the e-mail on the server mail-services.com to the destination xxx@gmail.com
(and not to the forwarding mail xxx@mail.ambis.cz).
Tip: Technical specificationThe sender's server administrator can set up from which computers his e-mails can be sent.
Therefore, the sender's server administrator can solve the IS AMBIS forwarding problem by entering the IS AMBIS address
into the list of computers from which it sends e-mails. This is unrealistic if the mail server does not have a close relationship with IS AMBIS.