Most organizations, in the effort of controlling SPAM from being sent out of their private network,
do prevent user from accessing the SMTP server port 25 directly.
To support customers with SMTP access requirements from a non-fixed location,
especially from private network with such policies, the following additional ports are available.
587 => startTLS/Auto (recommended for outlook and thunderbird clients)
465 => SSL/TLS on connect immediately required
25 => start TLS optional (not recommended unless you cannot use 465 or 587)
Customers are reminded to enable SSL/TLS encryption authentication even for
port 25 even though it is not strictly required.