i Was having mail issue after updating iPhone 6 to ios 8.3 with gmail and yahoo mail not sending or receiving on phone or Macbook Air mail applications. Saw a thread in another post suggesting some success after deleting and manually re-entering as other rather than default (yahoo example given). Air needed update to 10.10.3 so started that downloading. Went (on phone) to try the work around suggested. Here is what happened!
Went to settings ---- mail, contacts, calendars
1. before trying suggestion of deleting gmail I clicked on Gmail Account >
2. SMTP (Gmail SMTP server) >
3. SMTP page listed
PRIMARY SERVER
GMAIL SMTP SERVER -----> ON
OTHER SMTP SERVER
smtp.gmail.com ------> OFF (I clicked on this line)
4. next page I changed server to -------> ON
checked to see if e-mailing to myself through gmail would work and it did, interestingly e-mailing to myself through yahoo also worked
went back to settings and turned the same button back to the OFF position and gmail worked, yahoo didn't
once laptop update completed, the macbook air gmail worked, yahoo didn't
repeated above steps for yahoo to turn on other SMTP server and yahoo worked after turning on and still worked after turning off.
Phone seems to have shared with Macbook Air as it works as well (although turn around of getting the mail is a little slower than usual - particularly with yahoo). I'm not sure if this impacts sync of contacts/ calendars/ etc. Probably just got lucky.