It was used when timeout was set on the whole smtp.send() operation.
Now only the operations inside smtp.send() can timeout, and such timeout
errors result in SendError, so SendTimeout is unused.
15 minute timeout was used because it applied to the whole send()
operation. Now timeout applies to each I/O operation, such as read()
or write(), so it can be made much shorter. In particular, this timeout
applies to read() call right after connection to plain or STARTTLS server,
in which case it is not reasonable to wait 15 minutes to receive one
line of data.
The timeout is triggered only if no progress is made within 30 seconds. It
likely indicates a network failure anyway.
