Are you using a "strong" password? Email protocols don't support the full range of characters and some symbols which might be accepted by http as used in a webmail connection cannot be passed over the email protocols; restrict it to alphanumerics, and avoid punctuation, braces, asterisks and the like.

Thanks for the reply. My password is all alphanumeric and doesn't contain anything else. Any other thoughts?

I don't think it is good advice to tell people to use less "strong" passwords. All my email account passwords have special characters, and there is no problem with that at all.

What is your email provider?

Have you verified your credentials via webmail?

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I don't think it is good advice to tell people to use less "strong" passwords. All my email account passwords have special characters, and there is no problem with that at all.

The relevant RFCs indeed permit all printable characters to be used, but I have encountered more restrictive implementations.

The OP says he's "checked and triple checked that I am using the right password and user name" so I took it that his credentials work over the webmail connection. Let's see.