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I'm sorry about the issue. Is there any error or message when sending an email message?
When did this issue first occur?
Are you currently able to receive new email messages fine?
Does your email address end with @att.net?
Can you test going to the ATT.net website:
, then sign in your att email address if prompted, then see if you can test composing / sending an email message there.
Please type at the box below, then click on the Send button below to communicate with me, so I can help. Thank you,
Can you click on the Apple logo / menu at the top, then click on About this Mac..., then tell me your Mac OS X version, such 10.6, 10.7, ...., 10.10, 10.11, etc.?