I take it from what your saying that you mainly use Outlook Express and sometimes use MS Outlook. I also take it that your issue refers to MS Outlook. Now if this is true then you need to make sure that you have a profile set up in MS Outlook and that your pop3 address and smtp address are set up together with user ID and correct password.
Pop3 address is for send and SMTP address is for receiving. You have stated your ISP but for example if your ISP is NLT then the pop3 address would be pop.ntlworld.com and smtp address would be smtp.ntlworld.com and if your ISP is BT it could be for pop3 mail.btinternet.com or mail.btopenworld.com with the smpt address being the same.