Immediately after switching the page, it will work with CSR.
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It seems like Twilio played with this idea with the Twilio Dev Phone project[1] but that project doesn't seem to be actively maintained.
[0] https://mailtrap.io [1] https://github.com/twilio-labs/dev-phone
If something is important to someone in particular, they should implement a pull request or see the author's website about making a donation or paying for the development of a particular capability.
MailPit, MailCrab and smtp4dev are modern alternatives.
https://github.com/axllent/mailpit
Happy to answer any questions, or be persuaded that something is broken or would be a dramatic improvement :-)