I think this is because we had a problem with our corporate mail server, because:

a) I have a vague memory (but sadly no notes!) that it happened before, and b) in this case we had the problem when the mail server was knackered, it’s now fixed and the problem has gone away.

Make of this what you will

ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at "DBUSER.PK_ESERVICE_LOAD", line 496
ORA-20000: Unable to send e-mail message from pl/sql : ORA-29260: network error: TNS:operation timed out