If you want to create large Email newsletters with marketing offers, then the use of a proxy is a prerequisite. This trend has been observed for more than a year, so most proxy providers close mail ports.
We do not close mail ports (25, 443, ...). On all proxy lists, output IP rotation is available, which allows you to maintain a fresh IP pool during the day. Everyday we clean IP from spamhaus databases, which has a positive effect on bulk sending to a large number of addresses.