Gmail alias - dkim header not present

I’m using eM Client in combination with a gmail account, for which I’ve added two ‘Send as’ emailaddresses (not aliasses, both use Sendinblue as SMTP server).

In eM Client I’m able to send email using those aliasses. The thing is, if I send it with eM Client the spam score is only 6.5 out of 10: Spam Test Result

If I send a message from gmail using those aliasses, it all starts to look a lot better with 9.8 out of 10: Spam Test Result

The major difference is; using eM Client to send the mail is using gmail SMTP servers instead of Sendinblue and thus the DKIM stuff is incorrect or not present at all.

Why is it sent using the wrong SMTP server? How can this be fixed?