Sorry to hear about this problem. However I believe this problem you are seeing is actually a known bug in both 2.5 and 2.6 revisions of Sourcetree. This is documented in the bug ticket SRCTREE-5034.
I would recommend watching that ticket for updates as to when this will be resolved.
In the meantime, I found that a few other users have reported using the 2.1 version of Sourcetree in order to get around this documented bug. https://community.atlassian.com/t5/Sourcetree-questions/Sourcetree-not-accepting-email-address/qaq-p/647580
On both the bug ticket and that other thread @benjamin lemoineposted that work-around:
I hope this helps,
Supported Platforms macOS Windows To make using Sourcetree as simple yet powerful as possible we embed (bundle) dependencies such as Git, Git LFS, and Mercurial. We strive to keep these...
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