Most large Cloud Providers have an arrangement with at least one email service which allows some free service (at least no extra charge than whatever one is paying for the subscription), with a far higher limit than the typical free tiers - e.g. AWS/SES, Azure/SendGrid, IBM/SendGrid, GCP/Mailgun.

So if you're already paying for some cloud hosting somewhere you likely have a free email service you can use.

Disclaimer: I work with Twilio SendGrid.

We have the inbound parse webhook which turns incoming SMTP into an HTTP post to your URL. Our free plan is is 100/day after the trial period which has a much higher limit.

Gmail + a mail merge addon (like Mailmeteor.com or Gmass)

Mailjet, even though it is owned by Mailgun, has a higher free tier.

Sendgrid is owned by Twillio, and has a similar free tier.

Ses is pretty cheap, for 500 mails, it is essentially free.

Or get a dedicated IP, use mailinabox or other similar one click setups.

I Own our private VPS where I was sending and receiving without trouble in the past but currently I am not able to setup mail server correctly what is reason why I started to use mailgun.

Just curious if it is 500/month, why switch?

From the description, it seems, that you have just 500 mails to send. You can not receive emails anymore. Or am I wrong? Thank you.

This is downvoted, but correct. You can programmatically tap into a Gmail account and send 20-30 emails per day, which adds up to the 500/mo.

Whilst technically correct the issue with gmail is it overwrites the from address with the gmail address.