Port 25 is already in use, specify another port value (see section 'Cooperation with other software' in 'Manual Setting of Mail Protection' in help).
It is not too hard. Sending email (SMTP protocol) uses Port 25 as a default. If it is being used by another program, then you need to change the ports used in OE and within the Avast .ini file
Open up OE and go to Tools, Accounts.
Find your email account username and click on Properties.
Then go to the Advanced tab. Reset port 25 to 26 and port 110 to 111.
Now we need to change the Avast .ini file which you will find by going into the Programs folder/Avast/Data and find the .ini file
Open up the file by clicking on it. Within the file you will find the ports 25 and 110 listed. Change them to 26 and 111 respectively. Save the file and exit the whole mess.
Try the email again. This "normally" solves your problem.
Good luck.