About this question
I am looking for an email provider that I will use with a custom domain, one provider is cheaper but has only SPF while the other is more expensive but uses both SPF and DKIM and I'm not sure if paying more is worth it if the other factors are similar. I read about SPF and DKIM and I have some questions about them and also some about email security in general.
What benefit would DKIM provide if SPF is already used?
Is it correct to say that SPF ensures that the server from which the email claims to originate is the one from which it in fact originated and DKIM ensures that the content of the email message hasn't been modified?
If it's correct that DKIM protects the message from being tampered with, then from whom does it protect exactly? Who can alter the content of the email while in transit, what are the common attack vectors? Can there be a malicious "node" that relays the email but modifies its content?
Would it be possible for an attacker to make a server that relays emails and then snoop on them or modify the content, just like anybody can make a malicious Tor node?
If the email provider I use supports sending emails over SSL and I want to send an email to my friend who uses Gmail (which also supports SSL), does my message travel the whole path encrypted? Does it mean that the message is encrypted with a public key belonging to Gmail and no server which relays the message will ever see it unencrypted?
Let's assume that I use SPF. An attacker attempts to sends an email that appears to come from me. Where is this email relayed to (is this something called MTA)? Does it work like a node that forwards information to further nodes? If the attacker controls the first "node", can't he report to the nodes that it forwards to, that the message passes SPF when it does not and the nodes that get forwarded the message would believe it?
