I have noticed that when using DevelopersRecipientProvider in the declarative pipeline, the information returned is the userid and not the valid email address.  I get the error:
 
Error sending to the following INVALID addresses: abc543@company.com
abc543 is the user that committed a change. The real email address is jim.smith@company.com. abc543@company.com does not work. 
I turned on a log appender and this is the information I get

Resolving e-mail address for "Smith, Jim" ID=abc543
Jun 04, 2020 1:36:42 PM FINE hudson.plugins.active_directory.ActiveDirectoryAuthenticationProvider
Login successful: abc543 dn=CN=abc456,OU=Users,OU=EXT44,OU=IN33,OU=HQ,OU=COMPANY,DC=COM
Jun 04, 2020 1:36:42 PM FINE hudson.plugins.active_directory.ActiveDirectoryMailAddressResolverImpl
Email address = 'null'

I called code similar to ActiveDirectoryAuthenticationProvider and noticed that IADsUser.emailAddress() returns the valid email address so it would appear that the problem is in ActiveDirectoryUserDetail.updteUserInfo(). I say that because later the ActiveDirectoryMailAddressResolverImpl returns a null email address and I presume this is the case because it was not save correctly in ActiveDirectoryUserDetail.updteUserInfo().