I hope it can handle forwards.  The MX records have changed for example.com to point to the new email server. 

It just seems wrong that different users from different domains forwarding to example.com would be removed.  It seems the forwarding would be left alone if the forwarding was from completely different domains.

restore backup of vmail database to a new database of your choice, then:

SELECT * FROM vmailrestore.forwardings WHERE vmailrestore.forwardings.dest_domain ="example.com";

there you get all columns where the destination of the forwarding is example.com

export those to a sql import file (dont use the column id since it is unique and autoincrement) and import it to your actual database and all should work again

Edit:

@ZhangHuangbin

I agree that just deleting a domain should not cascade all references in vmail.forwardings that point to the deleted domain, since it doesn't matter if the forwarded domain is locally hosted or not, either remove that completely or make it a checkbox with a warning like "delete all forwardings pointing to that domain aswell" so ppl have the choice to do so if they want

This issue has been fixed moment ago, and will be available in next iRedAdmin-Pro release.
Here's the patch for iRedAdmin-Pro-SQL-5.0. iRedAdmin-Pro customers (with valid license) can contact us (https://www.iredmail.org/contact.html) to get a patched version.

diff --git a/libs/sqllib/domain.py b/libs/sqllib/domain.py
index f9dd4573..44fd0508 100644
--- a/libs/sqllib/domain.py
+++ b/libs/sqllib/domain.py
@@ -680,21 +680,6 @@ def delete_domains(domains,
                         vars=sql_vars,
                         where='domain IN $domains')
 
-        # Delete destination domain name.
-        for tbl in ['forwardings', 'moderators']:
-            conn.delete(tbl,
-                        vars=sql_vars,
-                        where='dest_domain IN $domains')
-
-        # Introduced AFTER iRedMail-1.3.2.
-        for tbl in ['maillist_owners']:
-            try:
-                conn.delete(tbl,
-                            vars=sql_vars,
-                            where='domain IN $domains OR dest_domain IN $domains')
-            except:
-                pass
-
         # Delete alias domain
         conn.delete('alias_domain',
                     vars=sql_vars,