Can't send mail

Hello TBUDL'ers,

I had to reload my wife's TB! onto a new HDD because the old one died. I had her
old v6.0.12 .msi file and her key block so I was able to get TB! up and running.
However, although she can receive mail she cannot send any. The log reports:

!4/6/2024, 18:45:11: SEND  - Server reports error. The response is: 5.1.0 <*******@charter.net> sender rejected

Although I was quick to blame Charter, I *was* able to send mail from her Gmail
account so something is amiss with TB!

Any ideas what I should be checking?

-- 
TIA,
Jack LaRosa

Using TB! 6.0.12
OS: Win 10 v6 Build: 9200

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Current version is 9.1.18 | 'Using TBUDL' information:
http://www.silverstones.com/thebat/TBUDLInfo.html