Email address &1 is invalid

What causes this issue?

You have tried to activate the current e-mail action. However, the
system has found formal errors in the recipient address '&v1&'. To help
you understand the problem, the system provides the following additional
error information:
&v2&

System Response

The e-mail action cannot be activated.

How to fix this error?

Make sure the addresses of the e-mail recipients are formally correct.
The following requirements may help you identify some frequently
observed errors:
Allowed characters: a-z, A-Z, 0-9, +, -, _, ., ~
Name part and mail server part separated by exactly one @
Mail server part contains at least the domain name, followed by the top
level domain (for example, "company.com", "university.ac.uk",
"firma.de")
If there are multiple recipients, separate them by a semicolon ';'.
In addition to the technical e-mail address, a comment part may be
supplied for each recipient. This can be used, for example, to convey
the recipient's real name. If you want to use a comment, the following
formats are allowed:
comment <<name>@<domain>>
<name>@<domain> (comment)

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.