Your phones might not have identical settings.
I would suggest to check a few settings:
1. Battery optimizations - read the FAQ item "Persistent network errors or the app not checking mail (any phone, with specifics for Android 6+, MIUI, Huawei)" that explains this in detail.
http://www.aqua-mail.com/?page_id=227 [nofollow]
2. Please check AquaMail settings -> network and make sure that the one that mentions "DNS" (second from the top) is *enabled*. But then it is enabled by default.
Who is your e-mail provider?
I actually cloud the app backup settings, so that whichever phone I use, it sets-up the app correctly and consistantly with correct email settings, etc.
It has worked flawlessly on Note 4, S7 Edge and Note 7 using Android 5.x/6.x. This issue has only happened since using Pixel XL/Android 7.x.
1. Will check this out....
2. iiNet, Gmail, Bigpond and work emails - all through wi-fi. Noticed this problem when only using wi-Fi.
Cheers.
Edit: OK...since I did the battery optimisation reset, it seems to be OK now with wi-fi mail check at home. Hopefully it will be good when I return to work tomorrow also.