iPhone and iPad email problems when connected to Verizon DSL

HI there,

My iPhone and iPad (both using 6.0.2) work fine when connected to the Verizon DSL router.  Both internet and emails are ok. 

However it won't send any emails that contains an attachment...like photos etc, no matter what size they are.

Basically it tries to send the message for about 5 minutes, then just sits in the outbox with an error message. "A copy has been placed in the Outbox. Sending the message content to the server failed."

Any ideas anyone?  Guessing it must be a Router issue as they send fine from the neighbour's DSL and over the verizon wireless network. It also occurs when connected to an Apple Airport that gets its internet connection via the same D-Link router.

(By the way this happens from 5 different email accounts..mac, google etc)

Thank you