Can I use an accountant's letter of income for a marriage visa application in Thailand?

Temporary solution while you build up 12 months of Foreign Funds transfers in your Thai Bank Account. (just have your foreign salary sent there each month). How to "buy" time... Go to the Thai Consulate in Savanakhet or HCMC Vietnam (with minimum marriage documentation, (no financial requirement) except for the Visa Fee. This O Visa one year Multi Entry does require you to do an "out and back in" border run every 90 days. And you can stretch it from 12 to 15 months. Now at the end of 12 months or so you will have 12 documented International Transfer Deposits into your bank account. Armed with that info and more extensive marriage docs you can go to a Thai Immigration Office where you live and apply for a One Year Extension of Stay based on the original O Visa for marriage to a Thai lady. And this Extension of Stay only requires reporting to the Thai Immigration Office once every 90 days.

you have an alternative: Go to Savannakhet in Laos or HCMC in Vietnam and apply there at the Thai Consulate for the 1 year multiple entry Non Immigrant O visa based on being married to a Thai National. On each entry you get 90 days of stay inside Thailand, then you make a border run for a new 90 day of stay, if good planned you can get 15 months of stay out of this visa. You do not need to show any financials other then the 5.000 Thb fee for this visa.

Depends on the country of origin. OR you use the income affidavit from your Embassy with a minimum income of 40.000 Thb a months, OR in case your Embassy does not provide this income affidavit, you use at least 12 months prior application date a monthly foreign deposit of minimum 40.000 Thb into your Thai bank account OR use tax and sso payments for the last three months if you work inside Thailand and copies of the work permit, OR you provide letters from the bank and a bank book showing at least 400.000 Thb 2 months prior the application date on your Thai bank account.