ipad setup for sending emails from non-plusnet account

[#1]

My son has always had problems sending emails from his ipad when he is connected to my plusnet wifi network. Receiving is not a problem.
I have setup outgoing as relay.plus.net, no security and port 587 with my username and password. They sometimes do send but can take 10 minutes or more. Wifi signal is strong , web browsing is fine.
What are we doing wrong ?
Thanks
Mike

[#2]

Sending should be almost instant. Sorry to ask what might be obvious but is your son sending a large attachment, sending to a large number of people, or sending several emails at once?

[#3]

Thanks for response - in answer to question, no. He experiences same problem if, for example,  he just send a simple "test" message to me.
I have followed the instructions on the Plusnet site,  - Email setup Iphone, Ipad  Touch and Ipad - the setup completes ok. For the outgoing server I have specified as suggested -
Outgoing Mail Server settings
    Host Name: relay.plus.net
    User Name & Password: These are optional, but if you want to send mail while on the move (e.g. while connected to mobile or wireless networks), we recommend filling these in" -

but no improvement in the speed  of sending was obtained.
Thanks and Happy New Year

[#4]

Not so much an answer, but a suggestion...
Is there a reason why your son isn't using his email provider's server settings?  There shouldn't be a need to change his settings to work on your wifi unless his email service is linked to a particular ISP and they don't allow sending from outside their network at all?

[#5]

re using his email providers settings that is one option we tried yesterday - set up his email id and password on his isp - but it still does not send
we have identified that he has the same problem on his iphone sending a simple test message to me - after 1 hour still not sent.
will try to access another wifi network  locally (BT)  to see if same problem exists