From a purely SEO perspective, I don't think there will be any negative impact from using the acronym URL for your email addresses and the longer URL for your domain — the only concern I might have would be that people might try to link to the acronym domain after seeing your email, and those links would be passing through the redirect, but that would probably be an edge case.
I do think that 25 characters is a very lonnnngg domain name. If it's your brand, you're likely to rank #1 for it without having the exact new brand as the domain name, so you'll be capturing that high search volume anyway. You also have to think about this as a long-term play: there's a lot of search interest for the two brand names together now, because of the merger, but that's not the best reason to select a new brand name or a new domain name, both of which are things you'd want to persist for as long as possible, well after the merger is old news.
I know it might be too late for this, but I'd echo GrouchyKids' recommendation that you rethink how you want to brand the merger of these two brands. High search volume for one term isn't a great reason. If you do decide that you still want that to be your new brand name, you might still want to explore shorter versions of the new brand for your domain name. A 25-character URL is going to lose out on some direct traffic, as it will be difficult for repeat customers to remember and type in exactly (something your company must already be aware of, since they're looking for solutions not to have to type out the whole thing themselves every time they send an email!). Good luck with the rebrand in either case!