I am getting the same error while trying to share a file to an external email address
My email is setup as type sendmail and i have the below in my /var/www/nextcloud/config/config.php. I did noticed some smtp stuff in config.php probably leftover from before but it is set to type sendmail, and test emails work perfectly using sendmail. Also internal shares also work correctly with sendmail
So I do not think it is necessarily an email issue but when i try to send to external share there is absolutely no entries in the mail.log at all
‘mail_from_address’ => ‘admin’,
‘mail_smtpmode’ => ‘sendmail’,
‘mail_domain’ => ‘secret.ca’,
‘mail_smtpauthtype’ => ‘PLAIN’,
‘mail_smtpauth’ => 1,
‘mail_smtphost’ => ‘mx2.secret.ca’,
‘mail_smtpport’ => ‘587’,
‘mail_smtpname’ => ‘ryan@secret.ca’,
‘mail_smtppassword’ => ‘secret’,