SMTP and sendmail used

We use MailGun for sending out emails. But for some reason not all e-mails use the SMTP-settings. The forgot/ reset password option and the welcome message are send thru SMTP/ MailGun but the invite e-mails and e-mail confirmation e-mails are send thru the servers sendmail server (I can find those e-mails in the maillog)

Any ideas?

Best regards,

Michiel