Algol Plus Ltd

But  button "Test" ( in section "Email") shows successful message!

1.  If you use XLS format   - open section "Email"  ,    checkbox "Append file contents to email body"   must be turned OFF.

2.  Some mail servers can require additional verification for  emails  with attachments.

So you should use SMTP plugin, like https://wordpress.org/plugins/wp-mail-smtp/