Are you using Outlook as your mail client, by any chance? There's a recent update for Outlook that seemed to break its compatibility with default Postfix settings.
https://marc.info/?l=postfix-users&m=166585838304041&w=2
The workaround seems to be to disable session tickets:
tls_ssl_options=NO_TICKET
In main.cf, or adding it as another -o option to the submission port in master.cf if you only want to change the config for SMTP Submission.