Cloud
Emails can't usually be sent directly from a cloud provider to the recipient as there is a risk of abuse that cloud providers don't want to deal with. When someone abuses emails by eg. flooding emails or sending malicious emails, the IP used for sending emails is logged and possibly blacklisted by "greater email authorities". This prevents emails from reaching the recipients inbox until the IP is cleared again of the blacklist. Here are a few email blacklist checking services to tell if a domain or IP address has been blacklisted: * https://mxtoolbox.com/blacklists.aspx * https://dnschecker.org/ip-blacklist-checker.php * https://multirbl.valli.org/lookup/ * https://www.debouncer.com/blacklistlookup We are however still allowed to use ports 465 and 587 freely for mail transfer but this always requires a mail delivery service, like Google Workspaces, Office365 or Sendgrid
Port 465 (TLS)
This is a port that has been unclassified by Internet Assigned Numbers Authority (IANA) for SMTP use and as such should not be used for emails anymore. As it was once classified as a mail sending port, some legacy applications may still only accept this port and so there may still be a reason for it's use.
Port 587 (startTLS)
an email gateway