Oracle 18c

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create table tab (ID,  EMAIL) as
    select 1,   NULL from dual union all
    select 2,   'mail1@domain.com' from dual union all
    select 3,   'mail1@domain.com; mail2@domain.com, mail3@mail.com' from dual union all
    select 4,   'mail1@domain.com, mail2@domain.com' from dual union all
    select 5,   'mail1@domain.com mail2@domain.com' from dual
5 rows affected

select id, email from tab


ID
EMAIL


1
null

2
mail1@domain.com

3
mail1@domain.com; mail2@domain.com, mail3@mail.com

4
mail1@domain.com, mail2@domain.com

5
mail1@domain.com mail2@domain.com


select id, 
    regexp_substr (email, '(\w.*?)(([[:space:],;/])|($))', 1, 2, null, 1) as second_email
from tab


ID
SECOND_EMAIL


1
null

2
null

3
mail2@domain.com

4
mail2@domain.com

5
mail2@domain.com


select id, 
    regexp_substr (email, '(.*?)(([[:space:],;/]+)|($))', 1, 2, null, 1) as second_email
from tab


ID
SECOND_EMAIL


1
null

2
null

3
mail2@domain.com

4
mail2@domain.com

5
mail2@domain.com

ID	EMAIL
1	null
2	mail1@domain.com
3	mail1@domain.com; mail2@domain.com, mail3@mail.com
4	mail1@domain.com, mail2@domain.com
5	mail1@domain.com mail2@domain.com

ID	SECOND_EMAIL
1	null
2	null
3	mail2@domain.com
4	mail2@domain.com
5	mail2@domain.com