A client (another company) wants us to send them info via email regularly (hundreds of emails a day). They requested emails because each email will create a separate ticket in their task management system.
The emails contain private information. We can set up Mandatory TLS (a.k.a. "policy-enforced TLS") on our mail servers to make sure the emails are encrypted in-transit.
My question is: would it be better, instead, to set up a mailbox on our server and have the client retrieve the messages from this mailbox via IMAP over SSL? My first thought was that this would be more secure, since the mail would not be transmitted between unknown intermediate mail servers, but then I remembered that TLS is the successor to SSL, which I presume means stronger encryption.
So which is more secure: sending the email to the client's mail server, via policy-enforced TLS, or having the client retrieve mail from a mailbox on our mailserver via IMAP over SSL? (Our server doesn't seem to support IMAP over TLS; not sure if that is a thing.)
For the purposes of this question, assume that we have secure mail transmission within each respective company: I'm just asking about securing the mail transmission between the two companies. You can also assume secure mail storage on the two respective mail servers.
There's a similar question about Secure third-party inboxes, but such inboxes would be accessed via a browser, presumably with a TLS connection, not via IMAP over SSL, so I don't think it applies here. The client will retrieve messages programmatically.
My instinct is that having the client's task management system retrieve the messages via IMAP over SSL is more secure because it has fewer "parts", so there's less things that can go wrong. What isn't clear to me is how much benefit there is to using TLS instead of SSL.