So many possible factors here.

Which version of Outlook is this or is it Outlook Express?

If it is Outlook are you using the Outlook plugin of avast or the Internet Mail provider?

Oh, which operating system are you using?

What kind of email account are you sending mail to? Is this a regular POP/SMTP account or is this a Hotmail account or another type of Webmail account using a third party program to send mail?

Please give us some more information to help.