FortiPhish Campaign SMTP AUTH failed: 535 5.7.3

I am setting up a FortiPhishing Campaign which I later need to start providing to our clients. Now I am testing it on our own company network. Domain is verifyed, SMTP Testing is responding successfuly and LDAP server is has given me accounts I have needed.

But after lounching the campaign I come to an error: "SMTP AUTH failed: 535 5.7.3 Authentication unsuccessful".

I cannot find any solution so what can I do to avoid this error?