Emails I send keep getting into recipient’s Spam folder in Gmail

I have a personal domain and I use a mail account on it (it’s not Gmail). Recently, all the emails I send (personal and business messages, not bulk) keep getting into the recipients Gmail Spam folder (I checked with several recipients).

Why is that and what can I do?

More info:

  • I checked and my domain is not in the blacklist.

  • When looking at the message in the Spam folder, it states “It’s similar to messages that were detected by our spam filter”. However, this is a personal message I wrote to someone – not a newsletter or a bulk message.

  • My sites and mail server are on a shared hosting service. But until recently I didn’t have this problem.