Re: "MacBook Messages can Receive but not Send"
Is it possible that the issue may stem from your "macOS 15.8" version:
ie: Apple support's updates frequently contain bug-fixes and security updates
You may want to check which "MacBook Pro 13″ model you have,
for the newest compatible MacOS version would be listed underneath:
See how to: Identify your MacBook Pro model - Apple Support
If your Mac model can't update beyond MacOS 15
What's new in the updates for macOS Sequoia 15 says MacOS 15.8.1 is the latest version:
and "This update provides bug fixes and security updates for your Mac".
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NB: If you choose to update: It is always wise to back-up your Mac first
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Else: If your Mac is already updated to the newest compatible MacOS version:
Linked article below provides lots of tips, including:
Can’t change the time or time zone?"
See what to do: If you can’t send or receive messages using Messages on Mac - Apple Support
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You may want to check for interference from third-party (non-Apple) apps:
Eg: VPN (if not provided by an employer) is notorious for causing various issues:
As turning it off does not stop it from still running in the background, the best option is to use an uninstaller from it's developer to get rid of it and associated files.
For much more info: See: If your device has network connectivity issues, check for VPN and other third-party security software - Apple Support
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Useful resource for macOS 15: Messages User Guide for Mac - Apple Support
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If more support is needed:
Apple provides a Messages Support website: (info. / support / contact options etc.)
Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support