I have had this problem on and off for years, but is there a fix for it?

I open Outlook and receive mail (I use 'Mailwasher' so any SPAM is deleted before I download it). All is fine. I then find, maybe 20% of the time, that having written a reply to a given message, when I try to send it, I can't, as Outlook says it can't find the server. Usually, but not always, this is cleared by closing Outlook down and trying again.

The wi-fi signal is strong (in any case I get the same with a LAN cable) and it's not a settings/accounts problem or restarting Outlook would do nothing (and I have no problem 4 out of 5 times). Why is Outlook saying it can't find the server (can be BT or Yahoo, doesn't seem to be server specific) but then all is OK half a minute later?

OS is Win7 x64..

Thanks for any input.

Martin