Seb,
Nice to meet you.
I took the English US test in order to reply to this - I'm from the UK originally.
Now, your project is interesting to me. I had a strategy call last night with a client and I'll tell you what I told him regarding his mailing list.
1. You've got a list of email address
2. Segregate if you can so as it's not too general a list but if it is that's fine too.
3. I recommend a five email sequence. I see you're an agency so here's how I might do it.
3a. Email 1 (day 0/1) VALUE - offer value in the form of free work/graphic/critique whatever. Personally I offer a free graphic to show I'm serious.
3b. Days (or emails) 2,3 & 4 Content that's USEFUL. Give useful, informative and preferably actionable content.
3c. Day 5/email 5 Make them an offer.
4. Rinse and repeat.
That will get you engagement. Again, the saying applies "The money's in the list". Depends on who's on your mailing list but a rinse repeat sequence like that should help lift out those who genuinely want to work with you.
Now, you've got a celebrity list, too... remember, a lot won't even check their emails so don't lose faith with that.
Much like this proposal to you I'm giving you actionable content UPFRONT to demonstrate I know what you're going through and I care. Hopefully you'll get something tangible from this in order to help you make a decision on moving forward, whether with me or yourself or another FL.
Cheers,
Kevin