['@dom1.com', '@ddom2.com', '@dom3.com', '@dom4.com', '@dom5.com', '@dom6.com', '@dom7.com']

 f.writelines(list)

@dom1.com@ddom2.com@dom3.com@dom4.com@dom5.com@dom6.com@dom7.com

@dom1.com
@ddom2.com
@dom3.com
@dom4.com
@dom5.com
@dom6.com
@dom7.com

 data = ['@dom1.com', '@ddom2.com', '@dom3.com', '@dom4.com', '@dom5.com', '@dom6.com', '@dom7.com']
with open("имя_файла", "wt", encoding="utf-8) as f:
    f.write("\n".join(data))

 print(i,file=f)

  
>>> import sys
>>> 
>>> lst = ['a', 'b', 'c']
>>> 
>>> g = (i + '\n' for i in lst)
>>> 
>>> sys.stdout.writelines(g)
a
b
c
>>>

 dom = dom.replace('@','')

 AttributeError: 'list' object has no attribute 'replace'

 for i in lst:
    print(i[1:], file=f)

  file=f

 data = ['@dom1.com', '@ddom2.com', '@dom3.com', '@dom4.com', '@dom5.com', '@dom6.com', '@dom7.com']
xd = [i[1:] for i in data]
with open("a.dat", "wt", encoding="utf-8") as f:
    for i in xd:
        print(i,file=f)