OE gradually slows down

This is almost always a sign of folders in need of compaction and/or a fragmented disk. Clean up your newsgroups, compact your folders, then run Disk Defragmenter (Start Programs Accessories System Tools).

550 - Relaying not allowed

This error message is coming from the mail server, not from OE itself. It means that the SMTP (outgoing) mail server did not recognize you as an authorized user and so rejected your message. Most ISPs configure their SMTP servers to recognize only those whose IP address is on the accepted list, meaning that you are connected directly to that ISP. If you use multiple ISPs, you are likely to encounter this problem. The solution is to change the SMTP server for that mail account under Tools Accounts Properties Servers.
Another possible reason for this error is anti-virus software that configures OE to use a local proxy for the SMTP server. Since scanning e-mail is not necessary to protect your computer, you should disable the e-mail scan in your anti-virus product, and then re-enter the correct name for your SMTP server.

Errors when sending messages outlook

This indicates missing or corrupt information in the user information for a particular account. Click Tools Accounts Properties and delete then re-type the user name and email address, even if they appear correct. If the problem persists, remove the account, close and re-open OE, then create the account again.