Jose,
You started using MailWizz a little after me and I've been following your posts. I don't want to be a troll on this post but having used some different services, I am offering my personal experience to tell you that I would bet my last $1 that encoding has absolutely nothing to do with why your emails are going to spam.
You have two issues: First, there's Amazon & Mandrill and there's everyone else. Nobody has the deliverability that Amazon & Mandrill have. I know there are other very well established services out there but they do not have the reputation that Amazon & Mandrill have and your emails are going to be treated like they are from an unknown source. (And I suspect that every provider is subject to accidental blacklisting where they have to break their backs to get those situations repaired.)
You absolutely have to make sure that your DKIM & SPF are passing when they get through to the other side - view the source code of the email and search for "pass" & "fail" and make sure that any fail is accounted for, even if the email reached the inbox. I have multiple accounts on multiple services to track this stuff.
But regarding Microsoft: I have had the displeasure of using every Microsoft email service over the years: Hotmail, Windows Live Mail, Outlook, Office 365, and Microsoft's ability to filter spam is non-existent. I have had dozens of occurrences where I sent a single email to someone in the same domain and had the email go to spam. My college used to use Windows Live Mail and now uses Office 365 mail and I constantly have email from administrators landing in my spam filter. There is just no possible logic that anything programmed to check for spam would not exempt someone emailing me from the same [private] domain! If I had to guess, I would say that the Microsoft filters are very sensitive to the sending rate. Microsoft has absolutely no credibility in what it delivers to a user's inbox and while it affects us as senders, people need to know just how much email they miss because of how badly Microsoft filters the mail. On the other hand, iCloud will discard your suspected spam with no notice to either the sender or the receiver. MANY ISP's do this too.
About the encoding, I can tell you that I won't claim to be an authority on this, but I have had to track down about 20 different incidents of missing emails over the past 2-3 years and spam filters/systems/blacklists are all absurdly simple to the point that it's offensive and you would be giving these guys way too much credit to think that they look at anything so technical as your encoding. Even if I'm completely wrong on my premise about Base 64 encoding, I would never believe in a million years that Microsoft would filter on that - what ends up in my spam filter is the most ridiculous spam - in fact, I don't actually get spam on those accounts, so every single item that Microsoft flags is a false positive. (I'm trying to confirm this information with people that I think might know and I can't - but I'm telling you that the filters for Microsoft are so awful that I just can't believe this has anything to do with what you were thinking.)
From what I can tell, no transactional email service has any way of reporting that an email was delivered but that the recipient's system sent the email to the spam filter automatically. As long as it accepted, it is considered delivered.
I mean no disrespect but your content, when translated to English, has wording that corresponds to much of the email I find in my spam filter from Phishing attempts. People take a religious angle when they are trying to get you to send them money to help their loved ones, etc. I think you're going to always have some trouble delivering your email. I have a similar situation with content not appropriate for under 18 year olds and we have no way really to know if our emails get into the inbox. My only solution here is to keep track of the open rates in a spreadsheet so I can track irregularities. If anyone knows of other ideas or tools available to track this information I would love to hear about it.
Thanks in advance.
