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The Mac OS stops being able to send mail, while the iPhone at the same time is able to send email using the same account/server and credentials. Receiving mail functions on both systems well.

What can cause this problem and even more important how can I solve it?

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Are the iPhone & Mac on the same wireless network or is the iPhone using its cellular data connection? – Doug Harris Feb 2 '11 at 20:51
Can you be more clear when you say "unable to send mail." Is there an error? – Josh K Feb 2 '11 at 20:51
The (Mac os x) keeps on trying to send the mail for ever (sponning gear) until I quit the app/cmd Q. Connection Docter reports that connection with the server is OK (green dot). – Fred Zelders Feb 2 '11 at 21:26
iPhone is on 3G while MacBook is on Wifi. – Fred Zelders Feb 2 '11 at 21:27
up vote 2 down vote accepted

Since your devices are on separate networks, it's probably due to restrictions placed by your ISP on how you send email.

For example, if your ISP is Comcast (in the US), they will let you send email using one of their SMTP servers -- likely something like They will not let you send email using an SMTP server from another ISP and usually block any traffic outbound on the standard SMTP port (port 25). This is done to prevent computers within their network from inadvertently becoming spam senders. The side effect is that you have to use their SMTP servers.

If you've verified that this is the case (check with your ISP's site or other support), here are some possible solutions:

  • User your ISP's designated SMTP server (google for the name of your ISP plus "outbound email server" or "smtp server" -- or the equivalent in your language of choice)
  • Check the documentation or help for the SMTP server you're trying to use for outbound email and see if they offer the ability to connect on a non-standard port. If so, configure to use that port instead of port 25.
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Thank you for your explanation. This is exactly what I think what is the issue. However first attempts to change the SMTP settings did fail: Not able to change the port number. After change and save and exit the app the port settings are back to the original settings. – Fred Zelders Feb 4 '11 at 14:19

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