I can take my domain, customize DNS records and in a couple of minutes I am using a new provider. They also allow to export email content, which means I obviously don't lose anything.
With a free email account, you are anyway locked-in as with every provider, because you are using their domain. You can set automatic forwarding in that case.
Vendor lock exists when you invest substantial amount of work to build tools around a specific platform (say, AWS), or where you have no way to easily take the data from one platform out and use something else to do the same thing (say, Meta).
The fact that you can't use SMTP, which is a protocol that requires data on the server is not a vendor lock-in in any sense of the word. It's a decision that depends on having that content e2e encrypted, because the two things are simy incompatible.
Also the code for all Proton clients and the bridge is open source, and the bridge is essentially a client that emulates being a server so that you can use your preferred tools to access the emails.
Even in this scenario, there is no vendor lock and all it takes is changing the configuration of your tool from the local bridge address to whatever SMTP server you want to use elsewhere.
Can you please describe in which way you are actually locked-in, to show that you have a clue about what the word means?