fix read and write I/O timeout.

[ly020044]

If the connected pop3 server port is not open, the timeout option will not take effect.

[knadh]

Thanks for the PR. This should ideally be new params called opt.ReadTimeout and opt.WriteTimeout. In addition, the timeouts should be reset on encountering ErrDeadlineExceeded.

[ly020044]

By reset do you mean when the io.Reader or io.Writer encounters os.ErrDeadlineExceeded? I don't understand, thanks！

[knadh]

After a deadline has been exceeded, the connection can be refreshed by setting a deadline in the future.

It is explained in the comments here: https://pkg.go.dev/net#Conn

Basically, if a connection times out, if it has to be reused, its deadlines have to be set again.

[ly020044]

Thanks for reply!