Postfix not honoring mx

[David]

Folks

I'm running Ubuntu server 22.04, with Postfix 3.13-0ubuntu1.2.  I 
notice in the logs (mail.log) that when sending mail it is trying to 
connect to the target server, not its mail server.  For example, mail 
addressed to
   some-name at sonic.net
produces the error message in the log:
   Nov 10 15:05:46 xxxxx postfix/smtp[1772824]: 1FBAA8C0518: 
to=<some-name at sonic.net>, relay=none, delay=30, 
delays=0.03/0.02/30/0, dsn=4.4.1, status=deferred (connect to 
sonic.net[209.204.190.64]:25: Connection timed out)

The 209 address is the address of 'sonic.net', but the MX record, as 
revealed by the command "host -t mx sonic.net" is:
sonic.net mail is handled by 10 mailin-02.mx.sonic.net.
sonic.net mail is handled by 10 mailin-01.mx.sonic.net.

whose IP addresses are in the range 157.131.224.(68-73).

What configuration option should I suspect?

Thanks

David