What should I use for a monthly newsletter instead?

I’d start with a service built for newsletters rather than general-purpose transactional email. MailerLite is one option mentioned for that use case. Check its rules for marketing email before moving the list over, since a new provider can also review or restrict an account.

The 150-message send may have prompted a review, but there isn’t enough information to know why the account was closed. Going over a free-plan limit doesn’t, by itself, tell you what the provider’s concern was. Keep the support correspondence and be ready to explain how each address was collected and how people can unsubscribe.

Before sending again, make sure these customers actually agreed to receive promotions, not just order-related messages. Keep a clear record of sign-ups, remove people who unsubscribe or whose addresses repeatedly fail, and make the sender identity and unsubscribe option easy to find. A genuine customer list can still have problems if people don’t expect marketing.

Compare services on newsletter features and support, not just price or sending limits. Some tools are aimed more at transactional messages, so confirm that recurring promotional campaigns are allowed. I’d also check domain authentication instructions and start with a small, engaged segment instead of mailing the whole list at once.