Although “only the first e-mail gets screwed up” is a new twist, one problem we’ve seen a lot with interaction link e-mails is certain security programs, which scan e-mail for links and visit those links looking for malware. We’ve done some things to try and guard against that, but it’s possible some over zealous scanning program is still managing to look enough like a person to make Discourse think the link has been used. Check your site access logs to see if there’s any requests to the link before your browser.