I have been using Lavabit for email for quite a while now. I love the simplicity, it works well, and it feels like an old school email provider before everything moved towards the cloud. When I setup my account I was able to assign a custom domain name for an alias. I figured this way, I can switch email providers without updating accounts everywhere.
Recently, I noticed that some of my mail has been ending up in spam. It was the worst when sending to Gmail users. I assume Gmail may have started to assign a higher priority to correct SPF records which is causing my emails to hit spam.
I have setup SPF and DKIM records for other businesses over my career and in this case I just forgot to do it.
The best way to check your own email is to view the headers on a sent email. The headers will show how the messages traveled across the internet and if it passed SPF.
The only way to resolve this is by adding a SPF record with your domain provider. I personally use Namecheap and the screenshot below shows the TXT record that I added to my account. I set the SPF policy to Strict (-all), however Soft Fail (~all) might be a better place to start.
After making this change, Google started to mark my SPF as Passed.
There are many resources out there for configuring your own domain. While this example is specific to Lavabit, it will apply anywhere. I might suggest starting with MXToolBox and their generator or checking with your email provider.