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I use postfix 2.5, but the following error is displayed, and emails collect in the email cue. Would you teach the cancellation method of this error?

lost connection with XXXXX while sending end of data -- message may be sent more than once
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The remote SMTP server is behaving badly by not properly acknowledging the completion of the SMTP transaction. The only time I've seen this in the wild is when the far end is using a content filtering anti-spam/anti-virus backend and it takes too long to complete the scan, or else the MTA doesn't handle "this is spam" results cleanly.

Assuming that you're not actually sending spam, then the problem is almost certainly at the remote MTA, and you should contact them to get the problem worked out. If you're seeing this message when contacting a wide range of remote MTAs, then something must be screwy with your network connection (or someone in the middle is playing strange transparent proxy games... "great firewall of china" style games).

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