I don't think that this is a problem of the client in the first place. The problem is, that pure SMTP (the protocol for sending mail) has no user authentification. So everyone can use SMTP so send mail via ANY Server. So some provider allow sending mails over their server only if you have first fetched your mails via POP. Fetching mail does only work with a valid username and password. If the server detects a POP-login it allows sending mails for this IP for a short period of time.
The problem with clients like MS Outlook is, that they always try to send outgoing mails via SMTP before fetching incoming mails via POP. So sending mail will only work on the second try. A solution is to create 2 accounts for the same server and then to use the second one for sending mails.
If it is possible with PHP to fetch mails via POP then this problem would be solved. But I don't know if it's possible.
nòóx
Downhill Rangers