We have upgraded to 18.0.45.1 and the error about sendmail has disappeared, however it is now replaced by another error:
554 5.7.1 <
[email protected]>: Relay access denied\".\" at /var/www/vhosts/domainname.co.uk/httpdocs/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php line 306