Let’s take an example: I have 100K email leads in the 'Make Money Online' niche, all collected personally over the past few months. What would be your advice on contacting them all at once?
If your budget is low, just do random VPS mailing.
If it's legit and the budget is medium go to Sendgrid or Amazon or Mailchimp or Brevo. Brevo also has the German "inbox garantuee" certification. Depending on how many Hotmail and Gmail addresses, add a generic warmp SaaS or do it on your own.
If your budget is high, buy your own /24 IPs and use KumoMTA/PMTA with a sender application like Sendy/IEM/AtomicMailer.
"Legit" is also always subjective. If you just upload your list to f. ex. Sendgrid you might get some questions asked (they are pretty cool tho). Mailchimp probably would just ban you without refunds. Best approach for ESPs which allow single optins (non-confirmed) is to feed like 100-200 per day to them with proxies and a landing page connected to their api so they really think its all legit.
Btw, I am doing a mix of all of that, Sendgrid, Amazon and a couple of /24s with PMTA and in the future KomuMTA and mostly use a customized Sendy version and AtomicMailer. I used SSE Enterprise for 15 years but then finally my disk crashed and the key wasnt working anymore.
You might be wondering why I haven’t been emailing them continuously as I collected them. The reason is that my system automatically disables leads that haven’t engaged in the past XX days (usually 21 days). I’ve found that this approach works best with my single IP VPS setups.
Email addresses never expire, unless quota or hard bounced. Removing emails which havent been opend the last 100 email campaigns might be an option but many email providers block the trackers and you might end up deleting emails which are active. I only delete based on no MX on domain, quota, hard bounce, spamtrap or unsubscribe, I would never delete because they are "old".