Hi,
Since your didn't mention it, I'd like to know if you use Outlook as your mail client? If so, please try troubleshooting the issue as per:
Does the sent mails show correctly in webmail access? If not, please also try sending an email from webmail to see if this issue continues.
In which mail format did you compose the email? Please try switching to another format (HTML, Rich Text Format, Plain Text) to see if this issue persists.
If you have any antivirus software installed, please try disabling the integration in Outlook and then test it again. Some antivirus software may integrate itself into Outlook and may scan your sent and received emails, which could cause some unexpected
problems. You need to contact the support of the antivirus software or refer to the documentation of the software to disable the integration.
In addition, we can also start Outlook in safe mode to check if this issue is caused due to problematic add-ins. To do this, press Windows key + R to open the Run command, type outlook /safe and press Enter.
Please let me know th result.
Regards,
Steve Fan
TechNet Community Support