ahmedsa
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OK so your sme server mail system (when using webmail) appears to be working OK, sme can send & receive locally & externally.
The issue is that sending to external addresses take a longtime (a day or so).
As you have set your ISP's smtp server to deliver mail via, then that means delays are probably happening at your ISP's mail server.
You need to speak to them to ask why it is taking so long for them (their mail server) to deliver your mail to yahoo & gmail accounts etc.
This does not appear to be an sme server issue.
You should more thoroughly check your log files to identify the time & date that mail was sent from your server.
Use a command like
tail -f /var/log/qpsmtpd/current | tai64nlocal
tail -f /var/log/sqpsmtpd/current | tai64nlocal
Look for the time in the logs when you actually sent a message on your sme server webmail system & see what happens to it.
You should also be able to see some of these time/date details in the header of the received message (at yahoo account), you can see the mail servers that the message passed through & the time & date the message arrived & was dispatched etc, & therefore determine how long it took (or waited) at a particular mail server (eg your ISP's) or even when it was really sent by your sme server.
You can see this english date/time format format when using server manager View log files panel.
You can try removing the ISP's smtp server address ie the SMTPSmartHost setting would be blank, which means your sme servers mail server will send directly to other mail servers connected to the Internet. In this case the integrity of your domain becomes under question, as other mail servers will run checks such as spf, reverse DNS (ptr) & other tests, & may filter or block mail from your server if the credentials of your server do not match strict standards (this is to prevent spammers using their own mail servers).
It is worth a try to send directly to other mail servers (and not via your ISP's smtp server), to see if mail delivery to yahoo & gmail is any faster, but you should read the following notes
http://wiki.contribs.org/SME_Server:Documentation:Administration_Manual:Appendixie about PTR & SPF records
& if you have problems you may need to get your ISP to configure some of these settings for you eg spf, ptr & so on.
This error is of concern, but which transaction does it relate to ?
Getting the log file output in date & time format will give more useful info & allow you to correlate with when you send a particular email message.
@400000005543aae52d4c9884 delivery 2648: deferral:
Connected_to_127.0.0.1_but_greeting_failed./Remote_host_said:_451_Upstream_SMTP
_server_not_available/