[sylpheed:33931] can not send email enywhere

[Abolfazl Rostamzadeh]

Hello,

I just installed Sylpheed 3.0.2 (win xp)
while I can send email using two other email client applications, Sylpheed continusly shows 

Error occurred while sending the message:
550 See http://spf.pobox.com/why.html?sender=...

how "pobox.com" control my sending emails?

I use port 587 for sending.
receiving is good

where is the problem?

Best Regards
Abolfazl Rostamzadeh