I think every other AWS interaction already uses v4, since D14978 (January 2016).

We currently use a small, mostly-pointless piece of third party code ("SimpleEmailService.php") to interact with the SES API. The simple fix here is to just replace it with first-party code using the common AWS infrastructure, which was desirable anyway:

• T13235: Replace "SimpleEmailService" with a subclass of "PhutilAWSFuture" that surfaces errors more clearly

An adjacent change which is also desirable -- but significantly more complicated -- is to also move to a first-party SMTP client. The SES API is, roughly, a little bit of metadata wrapped around an SMTP-formatted mail body (there may be some more specific name for this "raw mail" format, but I don't know it if there is one):

• T12404: Implement a first-party SMTP client

Progress can be made toward T12404 while resolving the signature version issue without going all the way: SES only needs the code to format the mail body properly, not the code to speak the rest of the SMTP protocol. So an ideal version of this change is probably:

1. Move the API call to a new "SESAPIFuture", completing T13235 and updating the signature algorithm.
2. Move SMTP body construction to a new first-party class, completing some of the work in T12404.
3. Test the changes concurrently.