Hello.
I am using namecheap’s email service (privateemail.com) but all my discourse emails are being sent to spam in the major email providers, namely yahoo.com, hotmail.com, except gmail. This is better than getting sent to spam in all three services, and even getting mails not even accepted in hotmail (which happenned to me in the past), however, not ideal. The ideal being getting in the main box of every mail provider. How can achieve that? Does mailjet.com have better deliverability?
You’re using that as your main outgoing email provider? Not too familiar but that looks like a personal email account / webmail service, rather than one intended for larger scale transactional email of the sort used for Discourse.
Strongly suggest using one of the recommended email providers from the install guide. Personally I’ve had good experience with Mailgun; they have a generous free tier and are a popular choice for Discourse users. Mailjet and SendGrid seem pretty good too!
One of the most important things that determines if messages get flagged as spam is the sending IP, and these dedicated transactional email providers will be better equipped to make sure they maintain a good reputation, and help resolve and issues that may arise.
Another problem with this service - since it is meant for personal email - is that if too many emails are sent out within a certain time period, your domain name will be flagged for abuse… and disabled for several days (5 to 7) for the first offense. You will not have any access at all to your domain(s). They actually flag the registrant’s name and any domains you have with that registrar are disabled.
(I accidentally found this out the hard way just by forwarding too messages from one mailbox to another mailbox I have in too short a period of time.)
You are much better off going with an actual mail service like those suggested.
This is actually very good advice. What amount is too many emails? I have a comunity with roughly 30 users. Is that at risk of getting flagged? I just learned my domain is in some blacklist for being “too new” or something, so I suspect the recommended services will reject my domain. An old domain of mine got suspended from mailjet for no apparent reason. “risk to deliverability of our services” or something.
Sometimes I feel the internet establishment does a lot to discourage the small guy or the new guy from starting their projects, with the excuse of “preventing bad behavior”.
I’m not sure how many messages I was forwarding, but it was somewhere around 100 or more. I was simply forwarding them to a mailbox with unlimited storage - the first one being near capacity. After stopping to get a cup of coffee I returned to find a pop-up saying my session had expired and I needed to log back in… which I couldn’t. And I couldn’t on any of my domains with the same first level domain from that registrar. It wasn’t until I checked Whois that I found out I (the registrant) was “disabled for abuse.” 
Eventually I was “restored” and thereafter limited myself to no more than 10 forwards an hour. 
They’re just trying to stop spam. You could probably contact support for your domain registrar and/or webmail provider and ask them what the limit is. They may tell you… or not. 