Mail Merge compatible with Thunderbird or can be made to be

I have drifted away from M/S Word as much as possible but did enjoy its mail merge with Outlook. Now, I am looking for a replacement to facilitate bulk messages from my charitable organization to its members and potential members. Due to fiscal constraints the mail merge operation must be done with a freeware software product. If there is another email client than Thunderbird that is already compatible with 6.3 Libo that would be O.K. also.