macOS Mail app takes ~60 seconds to send emails but only for one SMTP server
I only managed to solve this by changing the port that I was using on my mail server to send emails (I was using a non-standard port to bypass ISP filtering). When I changed to using TCP port 587 (the Message Submission port) the issue was resolved.
I'm still not convinced that Mail and/or macOS (or possibly my mail server) is doing something weird as other devices (iPhone and iPad) use the non-standard SMTP port just fine.