Emails marked as sent, but never received by my clients

Hello,
For the past several months my office has been encountering an issue where our emails appear as succeffully sent, but are not recieved by the recipients, or only be a select few recipients... We're a religious organization, so our emails are distributed to indiduals/personal accounts, not businesses (with the odd exception of some small business owners), so I'm not sure what sort of security issue could maybe be at the root of this? Any advice or help would be greatly appreciated!