In principle there is no technical intrinsic restriction in the SMTP protocol limiting the size or number of attachments.

The SMTP protocol, however, requires that errors be checked and everything is synchronized at certain points.

HCP allows special-purpose access to the repository through the SMTP protocol that is only used for storing email.

See the history of SMTP protocol.

This technique relies on spammers using custom software that ignores the SMTP protocol.

It is not SMTP protocol compliant.

As long as the letter fits the protocol, (i.e. stamp, postal code) the SMTP protocol will send the message.

This must be a local program, typically using the SMTP protocol to send outbound mail.

The SMTP protocol allows for temporary rejection of incoming messages.

The SMTP protocol requires connections to be closed with a QUIT command.