Some companies do a reverse lookup on incoming email.
Say you have domain.com. Your MX record (record that tells everyone one the internet where your mail server is) may point to 64.56.X.X
But when your email server sends mail, it may be coming from a different IP address. If Juno does a reverse lookup, they see that the email from domain.com is not coming from 64.56.X.X, so they could flag it as spam.
There's also a chance that your server is blacklisted. DNSBL (DNS blacklists) have the IP address of spammers.....your email server could be on that list.
Not all companies use reverse lookups or DNS blacklists......I have no idea what Juno or others use (I use 3 blacklists for my mail server, my spam dropped by over 85%)
To start, check your SMTP queues; if there are a ton of things in there, your mail server is an open relay. Also, do an nslookup on your domain's MX record and make sure it's the same as your mail server is set for (hopefully, your behind a firewall, so you'll have to check if it's statically NAT'ed or published)
Good luck!