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With the smarthost sendmail does an MX lookup first, like
nslookup -q=mx outlook.fqdn.uk
If there is no MX it falls back to gethostbyname lookup, like
getent hosts outlook.fqdn.uk
The second goes...

I guess the smarthost in /etc/mail/sendmail.cf (and/or submit.cf) is mailhost.fqdn.uk.
Check its DNS resolution with
nslookup -q=mx mailhost.fqdn.uk
Does it list many hosts? Are some of them not...