[Daedren]

What you want is a newsletter management / email marketing / mailing list program, not a mail server.

Selfhosting an email server has its challenges, and when you're using it to send what people may consider spam, those challenges only get much harder.

[lzzzam]

are there open source solution? Any program you would suggest in particular?

[kjs3]

You probably should be asking yourself how important is this to you/your company and how is it going to be resourced, because the operational overhead of running this is non-trivial, and certainly not set-and-forget. You technically can do it yourself, but there's a reason ESPs[1] exist.

[1] Email Service Provider, before someone complains.

[lzzzam]

Which esp would you suggest for a job board that need to send weekly and alert to thousand of subscribers?

Are there any free provider that are commonly used?

[kjs3]

Disclaimer: I used to work for one of the big ESPs, but not for any of the ones I'll mention. My old company is more on the enterprise side these days. Not an endorsement of anyone and all that...

Free? You get what you pay for. Like I said, consider how much is this worth to your business. Email is hard work.

I know a lot of people who use Mailchimp for this and seem to be generally happy with them. Also Constant Contact and HubSpot (the latter I know has a free tier of some sort). There are a lot of players these days, many new since I was in the business, and a search for "Email Service Provider Newsletter" will turn up a ton.