Mailings die when the mail server starts blocking the sender by IP. Proxies for mailings solve it: emails go from different addresses, and one block does not kill the whole campaign.
How it works: the mail client sends emails through a proxy, each session — a new address. This lowers the spam risk. But remember: proxies do not replace email quality. If content is bad — deliverability will be low with any proxies.
For mailings address cleanliness matters: if an IP is burned in spam, emails go to the Spam folder right away. Take proxies from pools the seller keeps clean.
Sending emails through SOCKS5 works with most mail clients — just set the proxy in SMTP connection settings.