Make sure:
1. The DKIM is set up.
2. The SPF is set up.
3. The Sender ID is set up. (Note: This is NOT the same as SPF!)
... and there's more records a person can set, but for only ~50 subscribers, I'd really not worry about them. Keep it simple, just do the basics.
For help with mail records, refer to
http://www.unlocktheinbox.com
Use the server that the site is on. Do NOT use a proxy.
Then use either
- plain text
- clean and basic HTML
- a simple newsletter template. Preferably NOT a free one, so spend $10 on
ThemeForest to get a good one that is spam-free.
- a "simple" (learning curve!) mail service like MailChimp, which has ready-to-use basic templates
Our servers send thousands of emails per month, so we monitor them very closely.
That should point you in the right direction.
Tips:
1. Set the DNS entries up at least 3-4 days before sending a mass mail!
2. Send yourself LOTS of tests, to make sure it's working.
3. Be sure to read message headers. You'll need to understand what going on there.