Error message when attempting to send email

[Anon732161]

I receive the following message when attempting to send email from my computer:

Message could not be sent to the following recipients: [recipient's address] (550 - 550 5.1.0 Authentication required

What is going on? what do I do?  It appears to have been sent by using my phone.

[Latoque]

Can you clarify------are you trying to send from the phone or the computer?  Why do you think it's being sent from the phone?  %00 series errors usually indicate a problem with the recipient's email address or account.

[Anon732161]

Hi,

 Because it bounces back from the computer, but I have received replies from the phone. I don't know what %00 means.  There are no problems with receipient's email, these are people and businesses i deal with frequently. I find that whenever there is a problem with email, it is inevitably Comcast's fault. Sometimes I have been driven to use my ancient AOL account which performs flawlessly, all the time.

[Latoque]

A typo on my part---------I meant to say 510-511 series error messages.