e-mail photos gets stuck in "outbox"

I've been having strange problems with my new iPhone SE. This is one of them.

I have a gmail account set up on my iphone. it sends and reads e-mail fine.

but when I try to send a photo from the photo app, it doesn't send the photos sometimes and seems to archive in the "outbox" until I open the mail app and look at the outbox:

1. open photo app

2. select photo I want to send

3. click the "send to" icon

4. choose "Mail", and put in e-mail address to send

5. tap "send"

6. choose the photo size (I've tried small, medium, large.. all same results)

6. I get a "whoosh" sound saying it was sent.

but occasionally the e-mail isn't sent immediately and is cached in the outbox. so after like, 10min (sufficient time to allow any slowness in the network), I check the Mail app, go to the mailboxes and look at "outbox", I see there is one unsent mail stuck in queue. but when I open the "outbox" I briefly see the e-mail with the photo attachment and then immediately clears.. THEN the recipient actually receives it.

This seems to happen more often when it's not on any WiFi network but only on cell network. but when I'm on the WiFi network, or when I turn off the WiFi on the phone, it seems to send photos immediately.

Any clue what may be happening?