The K2 host server is the one sending it.  So if you talk about security context, it should be the K2 service account.

 However, if you are talking about the sender's mail address, this can be configured.

The setting is stored in this file, SourceCode.EventBus.Server.config, under C:Program FilesK2 blackpearlHost ServerBin.

Change this tag

<sendmailfrom>System@K2.local</sendmailfrom>

Thanks for your response, 

I changed the tag, restarted the K2 service, but still not getting the mail.

I'm using a different server from K2 Blackpearl to send emails. Possibly I have to change some other tag(s)? Also, I'm working with roles in destination rules.

The mail server is relaying because I can send mails through default mail events.

Default mail events use the environment library "Mail Server" setting.

 However for the notification event, the setting is stored in C:Program FilesK2 blackpearlHost ServerBinSourcecode.Smartfunctions.Configuration.config.

Modify the smtpServer tag to reflect your correct SMTP server and then restart the K2 server service for the changes to take effect.

On a side note, the orthodox method would normally be to re-run the configuration manager and change the SMTP server value from there.

The SMTP Server tag was correct, I changed the smtpFrom tag to the correct email.

Still not working.

In SourceCode.EventBus.Server.config file the msmqpath and msmqerrorpath tags are referencing the blackpearl server and not the mail server:

<msmqpath>BlackpearlServerEventBus</msmqpath>
    <msmqerrorpath>BlackpearlServerEventBus Error</msmqerrorpath>

Should it be the mail server instead of Blackpearl Server?

Also on my Blackpearl Server I have the following configuration of Message Queuing:

Sub-Components of Message Queuing:
  - Active Directory Integration (Selected)
  - Common (Selected)
  - Downlevel Client Support (Not selectede)
  - MSMQ HTTP Support (Not selected)
  - Routing Support (Not selected)
  - Triggers (Selected)

Does this configuration matter at all? What message queuing must I use, Blackpearl Server ou Mail Server?

The MSMQ component is for the K2 server.  It is mainly the queue mechanism for the eventbus notifications.

Created a new notification event for the onworklistitemadded for a specific activity and it worked, so the problem is that the notification events generated by K2 (through the option in flow design: Would you like to send a Notification of the Event to the Destination User(s)) don't work.

As I see it, I have 2 choices:
- Fix/repair the automatic notification events (How do I do that?)
OR
- Create the notification events myself, but then how can I configure the users in roles as receivers of the email?

This sounds a bit strange.  Have you tried logging a support ticket on this?  K2 support might be able to investigate this a bit further to see what is happening with your notifications.