550 email error<... > can't create output

The person you are sending email to is over quota. That means that they have no more disk space left on server to receive mail. They could have received a lot of mail recently, or a few large messages, or they may not have checked mail for a long time, or various other reasons that caused them to not have any disk space left on server. You might try sending the message again at a later time.

If the infected mails are coming from the server

Please see that when mail travels from souce node to the destination node, it makes many hops in between, it doesn't travel in one hop and has to pass through many nodes in between.
If any of the node in the path is infected then your mail packets can get infected and carry the infection to you.
As the issue is not in our hands, we can only suggest you to use a suitable mail filter and antivirus.

Working with POP3 from Telnet

[deskmerc@Evil-Fish deskmerc]$ telnet xxx.xxx.xxx.xxx 110
Trying xxx.xxx.xxx.xxx...
Connected to xxx.xxx.xxx.xxx.
Escape character is '^]'.
+OK POP3 [cppop 8.0] at [xxx.xxx.xxx.xxx]
user username
+OK Need a password
pass xpasswordx
+OK You have 1 messages totaling 1618 octets from /home/username/mail/inbox (full load)
list
+OK 1 messages (1618 octets) [0 control message(s) hidden]
1 1618
.
quit
+OK Bye!

Testing IMAP using telnet

[admin@adp admin]$ telnet localhost 143
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
* OK Courier-IMAP ready. Copyright 1998-2002 Double Precision, Inc. See
COPYING for distribution information.
a001 login postmaster@mydomain.com *******
a001 OK LOGIN Ok.

Anything other than "* OK ..." is error.

0x8004210B / 0x8004210A

This behavior occurs because some Internet service providers (ISP) do not permit message routing through another SMTP server while you are connected to their network.
To resolve this behavior, replace the SMTP server for the other ISP or the other e-mail account with the SMTP server that is associated with the ISP that you use to connect to the Internet. To do so, follow these steps:
Contact your ISP to obtain the outgoing SMTP e-mail server address.
Start Outlook 2002.
On the Tools menu, click E-mail Accounts.
Under E-mail, click View or change existing e-mail account, and then click Next.
Click the account that you want to change, and then click Change.
Under Server Information, typae the name of your e-mail server that you determined in step 1 in the Outgoing mail server (SMTP) box, and then click Next.
Click Finish

How to get full email header for a email message

Full email headers tell an administrator the path that the message took on its way to your computer. It also can give the administrator information about the computer sending the message and the SPAM checks that the message passed through. To get full email headers you need to do the following for

Outlook Express
-------------------
1. Right click on the message.
2. Choose Properties.
3. Choose the Details tab.

Webmail
----------
1. Login to your webmail.
2. Open the message
3. Click on "View Full Header:
Outlook 2003
1. Right click on the message.
2. Choose Options

ThunderBird
--------------
1. click the + sign beside the subject in the message preview pane.
Copy the support headers and send them to your support person or enter them into a support ticket for analysis.

0x8004210B

--------
CAUSE
--------
This behavior occurs because some Internet service providers (ISP) do not permit message routing through another SMTP server while you are connected to their network.
---------------
RESOLUTION
---------------
To resolve this behavior, replace the SMTP server for the other ISP or the other e-mail account with the SMTP server that is associated with the ISP that you use to connect to the Internet. To do so, follow these steps:
Contact your ISP to obtain the outgoing SMTP e-mail server address.
Start Outlook 2002.
On the Tools menu, click E-mail Accounts.
Under E-mail, click View or change existing e-mail account, and then click Next.
Click the account that you want to change, and then click Change.
Under Server Information, type the name of your e-mail server that you determined in step 1 in the Outgoing mail server (SMTP) box, and then click Next.
Click Finish