If you’re sending from a new domain, you should “warm up” the domain, since mail servers are generally suspicious of new sending IP addresses. You can warm up a domain by slowly increasing the number of emails you send from the address for the first few weeks. Click here for Sparkpost’s guide to warming up your domain.
If you’ve been sending from your own domain for more than a few weeks, it’s likely already warmed up. After that point, the number of contacts you email at once shouldn’t impact your deliverability.
Note: If you’re sending from our default domain (jiltmail.com), no need to worry - this domain is already warm!