Resolved Emails received as spam, using AcyMailing

Hello!

I have to send the same email with AcyMailing to the new users after they have activated their account on the website. But the problem is that they mostly receive this email on their spams, even if I do the spam test before sending it (I even do this spam test 3 times, but it doesn't change much).

I have the same problem with the automatic email confirmation sent to activate the account (many users didn't activate their account yet because they don't see the email confirmation). The users don't have the reflex to check on their spam (I have to remind them every time to go see their spams).
I thought that making my website secured (https) would have fix it, but it didn't.

Is their a solution the make sure that each email I send is received normally?

Thank you for your help.