如果在程序里加了SMTP Relay Server 就可以使发送速度变快。我copy了原因：
If the SMTPRelayServers collection is empty then the email message is sent directly to the destination server, so a DNS server is required to determine where the message is to be sent. However, if the SMTPRelayServers collection is not empty then the message is sent using the specified relay server, and the DNSServers collection is ignored since the relay server is responsible for determining where the message is being sent to.
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