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,
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