Are you using smtp.purdue.edu to send email? Typically, users of Thunderbird, AppleMail, or third party email like GMail or Yahoo set to send-as your @purdue.edu address will use smtp.purdue.edu as your outgoing mail server.

Due to last week's spam service change, some people are no longer able to send email successfully.

There are two possible causes for this:

1. smtp.purdue.edu is not set to use Port 587 (TLS).

2. Your home ISP is using a set of IP addresses that are known to send spam. This could happen, for example, if your neighbor down the street has a compromised machine.

The solutions to these are straightforward:

For #1,  if you are using port 465 (SSL), change that to 587 (TLS).

For GMail, look for the gear icon in the top right, go to Settings, Accounts and import, and Edit your <send mail as> account to update it.
For Thunderbird, right click on your account, go to settings, Outgoing Server, and change the port to 587.

For #2, use VPN to connect to Purdue's network, so you will bypass the home ISP network settings - since there's nothing that Purdue can do to fix your home ISP networking problems. You might want to call and let your ISP know that you ran into this problem, though.