Possible to add non-JIRA users to a custom field incrementally?

[Christopher Linley]

Is it possible to add non-JIRA users to a custom field incrementally as they send issue comments via e-mail? So for example:

(1) We have a custom JIRA field NonJiraUsers on an existing issue
(2) user1@nojira.com sends a comment via e-mail and NonJiraUsers gets updated to "user1@nojira.com"
(3) user2@nojira.com sends a comment via e-mail and NonJiraUsers gets updated to "user1@nojira.com,user2@nonjira.com"

This way, non-JIRA users can be added as "watchers" of an issue when they make a comment.

Or is it possible to overwrite the existing field? E.g.

(1) We have a custom JIRA field NonJiraUsers on an existing issue
(2) user1@nojira.com sends a comment via e-mail and NonJiraUsers gets updated to "user1@nojira.com"
(3) user2@nojira.com sends a comment via e-mail and NonJiraUsers gets updated to "user2@nonjira.com"

I.e. we replace the old field value with the new value?

From what I've seen so far of Sender processing, JEMH will only populate an issue field if it has not been populated already.

(And I also wanted to say thanks for the help so far - I've been asking a lot of questions recently )

[Andy Brook]

Let me start at the beginning.

Expected Pass Case (default)

1. Unregistered (no JIRA acct) email user sends mail to JIRA from user1@nojira.com
2. Issue ABC-123 is created, user1@nojira.com gets a created notification
3. user1@nojira.com replies to the ABC-123 notification, adds another CC: to another email user user2@nojira.com
4. As user1@nojira.com is already a participant, they are allowed to 'bring in' new participants like this, JEMH adds user2@nojira.com to email user participants.
5. JEMH notifies both user1 and user2...

Expected Fail Case (default)

1. Unregistered (no JIRA acct) email users sends a mail to JIRA from user1@nojira.com and refers issue key ABC-123
2. Issue ABC-123 doesn't have user1@nojira.com as an email user participant, so JEMH rejects the mail,and ignores and additional email recipients.

Interesting extension case

1. Adding JIRA users to the mix, they have rights to the issue if JIRA permits it, but:
2. Where users do not have permission, JEMH can also be configured to treat unprivileged JIRA users as email only users in projects they cant normally create in.  
3. What this means is that users who do not have rights in  a project can also be 'added' by JIRA users, and those users can then interact by email alone, and be notified of further updates subject to comment security etc.

JEMH will only ADD email recipients to the participants custom field.  The sender field only reflect the original sender.