Email Attachment issues over O2 4G and Zen Internet

[DKING]

Good afternoon,

I am about 7 hours into one of the most infuriating IT issues I have had in years and all parties are pointing the finger at the other and denying all/any responsibility. What happens is this: If I try sending an email from my Zen hosted domain account via O2 4G data (Nighthawk router) with an attachment over about 100kb the email just sits in my outbox and never leaves. 50kb attachments go no problem to the same address. This is true when using both Spark and Airmail clients and is repeatable ie it does the same everytime. I can send the same 150kb attachment via a Fasthost domain email account over the same O2 4G connection and it works fine. Zen claim that when the email is sitting in my outbox they can see no communication from between my computer and their server at all. O2 claim that they have no way of monitoring data packets at all (really?!) and that all they can see is yesterdays total data usage. O2 also claim that there are no restrictions on access to the Zen mail server from their network and that the problem therefore isn't anything to do with them. It seems to me that the restriction must be in the O2 network somewhere as the data isn't getting to Zen and so the failure is not due to the Zen server rejecting the email. I am now in a situation where I have to swap over my network connection to a woeful Zen ADSL (currently running at 467kbps! ie less than double what I got on my last dial up modem 20 years ago!) in order to send emails with attachments and then swap back to the O4 4G connection if I want to access my accounts on Xero. Hardly a workable long term solution. Has anybody experienced similar issues or have any suggestions as to what may be going wrong? All help very gratefully received! Regards, Darren

[madasaf1sh]

@DKING 

What port are you using for the SMTP connection is its 25 , or 587 or 465 and is it over SSL or not? 

What happens if you change the port to 26? As some ISP's don't like Mobile Data Initiated connections to hit SMTP servers on port 25 as its a risk from open relay abusers.. 

What OS Are you using, as the packet trace needs to be done from the client and only from a desktop so Windows, Linux, or MacOS. and you need to use Wireshark to do this.  

What DNS settings are you getting and whats your Public IP when using the router?

What mail server settings are you using?