With email, there is an inbound server and an outbound server. The outbound is referred to as an “SMTP” server. (Simple Mail Transfer Protocol) Usually these are the same server, however it’s entirely possible that they are not. It’s also entirely possible (and quite likely) that they may be relying on their SMTP server from their ISP. It’s not uncommon for there to be a series of relays involved in the process. The only reason I can think that your carrier would matter is if they may be attempting to use an alternate Server than they normally do, and that your carrier is strict about not receiving mail from servers that aren’t validated as being the correct source for the email domain that they are sending from. It seems common anymore for the incoming mail servers to validate that the outgoing server the received from correctly corresponds with the mail it’s sending. This usually helps to reduce spam by effectively blocking all mail from unregistered “SMTP” Servers.